Welcome to Librarium Online!
yo is it good enought to put 3 suports with d-cannon into the game
and if yes can the defender squad join it???
Nope a d cannon cannot be joined by a defender squad because only independant characters may do that. Things like starcannons platforms can be Attached liek the codex says but as this aint the case with support you may not use them.
Also D cannons have a too short range ussaly yo ucan only shoot 1 or 2 times before being charged. Also you need to play against a CC army so the units need to come forward otherwise they will not get anything within rrange (or you n to walk but that aint smart)
However with that statement out of the way, I would say listen to ArchonBjorn. He has a really good tendency to not say stupid things and give bad advice.Learn from yesterday.
Live for today
Hope for tomorrow