The answer is; TR3s have the steering rack in front of the front axle centre line, thus the calliper is behind the centre line (facing backwards), because the norm on an suspension upright is that the calliper and steering arm are on different sides, forward/backward.
Which brings us on a very interesting observation (imo), which is that the Ace is somewhat unusual for a front-engine-rear-drive sports car in that the rack/steering arms are behind the axle centre line. Whereas other vehicles such as TRs, Healy Sprite & 3000, MG Y type A & B have the steering arrangement in front of the axle CL. The Ace architecture locks the design into a very rearward engine position. I wonder therefore whether this was driven by the choice of the front suspension uprights, were the uprights borrowed from another vehicle?