For me, as I said earlier it's a combination of reasons why they will keep 3 TEs but keep in mind one of them will be LaCosse and he is a "move" TE (IMO a big WR as he played a ton in as a slot WR as well)) so in essence they will only keep two true TEs who can in-line block. I'm guessing it's gonna be Uncle Ben and ASJ as the "in-line" TEs but who knows.
The other reason is - and I agree with you- is they want to de-emphasize the TE in the passing game (if they still had a prime Gronk that would not change) but they still want the schematic, game-plan flexibility, unpredictability element. Having ASJ and Ben on either side gives them that.
Certainly they can get by with only two TEs on the roster and have Dev be the TE3 but with LaCosse also a decent STer (102 snaps last year) he has value aside from being just a move TE.
So net-net, if we want to look at it from a depth chart and not a roster standpoint...
7WR
JE
Harry
Someone
Someone
LaCosse
Someone who can play in the slot
Slater
6RB
Sony
Burk
Harris
White
Bolden
Develin (FB, short yardage RB)
5TE
Watson
ASJ
LaCosse
Develin
Big fat guy