The following is what I have been doing for my assistance exercises for the past year and it has been working very well. It incorporates a few of the concepts that BrickHead and Stu mentioned, while still being slightly different.
Using your example of 3 sets targeting 8 reps, I would use a rep range of 6-8 reps, and on the first two sets I would only go for 6 reps, then I would push for 8 reps on the third set. If I was able to get the full 8 reps on the third set, then the next time I repeated the exercise (the next week, assuming you’re on a weekly schedule), I would keep the weight the same, but go for 7 reps on each of the first two sets and then again push for 8 reps on the third set.
Alternatively, if I was not able to get the full 8 reps on the third set of the first week, then I would stick with going for 6 reps on each of the first two sets. Only increasing the number of reps on the first two sets once I was able to get 8 reps in the third set.
Once you have managed to get 8 reps in all three sets, then the subsequent week you increase the weight, but drop the number of reps in the first two sets back to the low end of the range and start working your way back up to 3x8.
Week 1: weight =225 lbs
set 1: 6 reps
set 2: 6 reps
set 3: 8 reps
Week 2: weight =225 lbs
set 1: 7 reps
set 2: 7 reps
set 3: 8 reps
Week 3: weight =225 lbs
set 1: 8 reps
set 2: 8 reps
set 3: 8 reps
Week 4: weight = increase weight to 230 lbs
set 1: 6 reps
set 2: 6 reps
set 3: 8 reps
Week 5: weight = 230 lbs
set 1: 7 reps
set 2: 7 reps
set 3: 8 reps
and so on.
Remember that if you fail to get the targeted 8 reps on the final set in any given week, you repeat the workout without increasing the reps on the first two sets. This scheme seems to allow for continuous steady progress at a rate that’s providing me with enough time to adapt physically to the slowly increasing stress without burning out the nervous system by continually trying to force it to adapt to heavier weight too quickly. I also find that by initially only going for the upper end of the rep in the last set, I manage to get in quality reps with few grinders.
If you want to give yourself more time to adapt to a given weight, all you have to do is widen the rep range; for example, increasing the rep range to 5 to 8 reps adds an additional week with a given weight.
Obviously there are a lot of ways that will work; the above is just one method that I’ve found works for me mentally and physically.
Best of luck.