I couldnt find this posted anywhere, and I know it was rumored, but I just bought the TF Official Novel and it says and I quote -

''In a trice, Arcee's parts leaped acrobatically, combining into a single tall shape''

So accoriding to the OFFICIAL novel, the 3 arcee bikes DO in fact combine together!

And if this is old news (I know it was RUMORED) I do appologize for posting it again, and also whenever they talk about Arcee in the first chapter before the combining part, they always refer to them as the three components or they are all one and the same being.