Some of people used 3mm (see first picture), others applied 2mm for the two narrow pads on the sides. Probably both works well if you want to go for a full replacement.
EVGA GeForce RTX 3080 FTW3 ULTRA GAMING
VRAM front: | 85x45x2mm |
VRAM back: | 85x45x3mm |
Pads on the side: | 1mm, 3mm (or 2mm) |