If in the UK then Atlasworx is the way to go. MMbeatle in Wales has them in stock.
There are two versions of the AICS mag System for the T3. Initially they made what seems like a copy of the US CDI floorplate which came out before the CTR was invented. We helped Atlasworx come up with a version that has the same footprint as the original alu CTR floorplate but using aics mags. This is the one to go for as the pillar height is the same as the original Lite plastic floorplate making fitting a little easier if converting from Lite to CTR/AICS. On the Website choose the version with CTR in the name. MMbeatle can help.
Rifle stocks and the pillar height vary, especially if epoxy bedded. We asked Atlasworx to make these floorplates with a slightly too long mag release lever. This has the advantage that the magazine fit can be adjusted ... perfect. Not too loose, not to tight and help with reliable feed.
When fitting these mag systems, we prefer to have the action side epoxy bedded before starting to fit mag system. Then have the pillar height in such a way that the original action screws are say 0.5-1mm shy of going through the thread. Just to be sure the screw doesn't strip.
Mag release lever will need to be shortened by about 1mm however they all need to be hand fitted to be perfect. The angle of the lever engaging against the magazine should be in such a way that it doesn't ramp away when the magazine is pulled or under recoil, rather have it pull the lever in tighter when under pressure.
edi
ps. always test fit/fit mag systems with bolt removed.