i've wondered that for a while now too.
My thinking is strawberry goes well with both vanilla and chocolate. Like in the ice cream sandwich. To me, it's suggesting that Mine goes well with Take and Hiori he's their strawberry. But this is how I view it.
Fun fact: It's actually a reference to Neapolitan ice cream. I'm sure you've seen it before. It's well known for always containing three layers of different favors- chocolate, strawberry, and vanilla. Take any one of them away and it isn't considered Neapolitan ice cream anymore, just like how you can't split the three characters apart.
I noticed that the title is the same as the three flavours of ice cream that was bought. But I do not understand the signification behind the name or the demand that three of the same flavour should be bought instead. (=・ω・=) Can someone explain to me?