What is the plural form of "the name of which"?

Consider we have a sentence:

I have a box named "A".

rewritten as such:

I have a box, the name of which is "A".

How should we rewrite the sentence if now there is more than one "item"?

I have three boxes, named "A", "B", and "C".

Is the sentence below grammatical? :

I have three boxes, the names of them are "A", "B", and "C".

Solution 1:

It is still "which" in the plural.

The following sentence is grammatical:

I have three boxes, the names of which are "A", "B", and "C".

In your last sentence, you should replace "them" with "which"; so it becomes,

I have three boxes, the names of which are A, B and C.

Or, you could simply say,

I have three boxes, namely A, B and C.

I have three boxes, named A, B and C.

I have three boxes, A, B and C.