I think it came up earlier whether TR6 brake rotors were useable on a TR3A...
Although the wheel stud pattern is the same on both cars, the actual part
number is different. (Oh, and since the disc bolts directly up to the hub--it
doesn't just slip over the wheel studs--the wheel stud pattern isn't really an
209327 covers TR4 - TR6
203189 covers TR3-3A
While this doesn't guarantee it can't be done. I'd check carefully before
spending the $$.
The calipers are most certainly different. (Four or five varieties from late
TR3 to late TR6)