1) There has been a funeral, two weddings and three thefts since you left.
OR
1) There have been a funeral, two weddings and three thefts since you left.
2) There was a boy and two girls inside the room.
OR
2) There were a boy and two girls inside the room.
3) Two girls and a boy was murdered.
OR
3)Two girls and a boy were murdered.
To understand the solution to your problem, it is necessary to examine the word "there"
"There" is only ever an adverb. There is a common misconception that "there" can be a pronoun.
There means "at that place." (Here means "at this place" and "where" means "at which place")[1].
We know that there is an adverb because there cannot be the subject of the sentence:
There is a dog in the garden (A dog is in the garden.)
There are two dogs in the garden. (Two dogs are in the garden.)
because, in English, the verb's form is dictated by its subject, the subject cannot be "there", the subject must be "dog" and "dogs" respectively.
The inversion of subject and verb is relatively common when a sentence is fronted by an adjunct:
Only after the war was over did he go home
Dearly do I love you.
Many are the times that I thought of you. (Many are the times -> The times exist numerously.)
This explains the apparently irregular position of the verb
There may be
(i) weakly demonstrative and merely indicates the existence of an item -> “There is a dog in the garden.” Or “There are lions in Africa” This is because the only way that the verb “to be” can take an adverb is in its meaning of “to exist”[2].
(ii) clearly demonstrative when used with a non-stative/dynamic verb: "Look there!"
It might seem that there can be a noun/pronoun meaning “that place [over there]” e.g. “He left there last night.” This occurs only because the other forms - thence and thither - which indicate motion have been all but lost and “he left there last night” = He left from that place last night, in which “from that place” is adverbial and has the meaning of “thence[sup]1[/sup]”.
The question then arises of what the subject is in the example sentences:
1) There have been a funeral since you left, as well as two weddings and three thefts.
This can be understood as
(i) a funeral has taken place, and also two weddings and three thefts since you left.
(ii) a funeral, two weddings and three thefts have taken place since you left. (This is and extended meaning 4 of “to be” at [2]
2) There was a boy and two girls inside the room.
Likewise:
(i) A boy was in the room as well as two girls.
(ii) A boy and two girls were inside the room.
[1] There, here, and where also may substitute for thence, thither and hence, hither, and whence, whither.
[2] In Old English, there were three (maybe four) “to be” verbs: 1. the verb (sind/sindon and sie) which gave us “am, is, art, and are” (sind and sie) “I am John.”
the verb (beon/buan) that gave us “be and being”: this verb meant to become or to grow. “He is tall.”
the verb (wasan) that gave us was/were. This verb had the sense ‘to remain, to stay (the night) “I was in Paris,” and thus to exist, i.e. be present, (in a place or state) “There is a dog in the garden.”
(There is a fourth verb “earan” which has the meaning of “to create or come into existence”, but this is not conclusively connected to any of the current forms of “to be”.)
https://forum.wordreference.com/threads/there-is.3629911/#post-18496185
Further information can be obtained from the OED (behind a paywall)
There (adv.)
- Used unemphatically to introduce a sentence or clause in which, for the sake of emphasis or preparing the hearer, the verb comes before its subject, as “there comes a time when, etc.”, “there was heard a rumbling noise.”
In interrogative sentences there comes between the verb and subject, as “Breathes there the man, etc.?”, or follows the first word of a compound verb, as “Does there breathe a man?”, “Shall there be any notice taken of it?”
The same order was formerly observed after an introductory adv. or clause, as “Then came there a voice, Soon shall there arise a prophet.”
Grammatically, there is no difference between “There comes the train!” and “There comes a time when, etc.”; but, while in the former there is demonstrative and stressed, in the latter it has been reduced to a mere anticipative element occupying the place of the subject which comes later.
Preceding or following a main verb, or following any verb, there, thus used, is stressless (proclitic or enclitic: e.g. there-ˈcame, ˈbreathes-there, ˈis-there, ˈwill-there), but preceding be or an auxiliary, there has a slight stress, and the verb is enclitic (e.g. ˈthere-is, ˈthere-was, ˈthere-will).
a. with intransitive verbs.
c888 Ælfred tr. Boethius De Consol. Philos. iii. §1 Þa com þær gan in to me heofencund Wisdom.
1857 H. T. Buckle Hist. Civilisation Eng. I. vii. 399 From all these things there resulted consequences of vast importance.
c. with a verb in the passive voice.
1912 N.E.D. at There Mod. Here, there were found various relics of Franklin's expedition.
d. especially with the verb to be: cf. be v. 1b, 4b there is, there are, are equivalent to French il est, il y a, German es ist, es sind, es gibt, Spanish hay. (For such phrases as there is no saying = ‘it is impossible to say’, see no adj. 4.)
c893 tr. Orosius Hist. i. i. §22 Þær is mid Estum an mægð.
1823 F. Clissold Narr. Ascent Mont Blanc 22 There being no moon.
e. When a relative clause follows, the relative pron. (that, who, or which) is often omitted. Now chiefly colloquial or archaic, as in ballad style. Cf. that pron.2
?a1366 Romaunt Rose 1239 Ther is no cloth sitteth bet On damiselle, than doth roket.
1912 N.E.D. at There Mod. colloq. There's a man at the door wants to see you.
**5. a. At that point or stage in action, proceeding, speech, or thought; formerly sometimes referring to what immediately precedes or follows: at that juncture; on that; on that occasion; then. **
a1400 Relig. Pieces fr. Thornton MS. 77 *At myn endynge..I pray þe lady helpe me *þare.**
1706 G. Farquhar Recruiting Officer i. i. 2 Brother! Hold there Friend, I'm no Kindred to you that I know of, as yet.
