I'm wondering that's the main difference between an APO doublet and a triplet in terms of image quality (considering the aperture is equal).
There is not an easy answer to this question.
I will just say that the performance in not just about how many lenses, but also the kinds of glass used.
Here is an aproximate ranking of how several 4, " f/7.5 telescopes using two or three lenses would rank (assuming optical quality and coatings were the same, which they never really are but this is just to compare).
In last place would be a doublet using non-ED glasses. This will have the most color on bright stars and planets will have visible chromatic aberration. There will be a pale violet non the surface of the moon and Jupiter, and you may see a heavy fringe n the limb of the moon.
Next up would be an FPL 51 doublet. This will have much less color on the same targets, but will be an improvement on the non-ED doublet.
The next type would be an FPL 53 doublet. This is a very exotic (and expensive) kind of glass with better dispersion characteristics than FPL 51 (or equitivlent glasses).
Now all the hype is about ED glass, but here is the reality.. A third lens is a very powerful tool in controlling chromatic aberration.
At 4", and f/7.5 you could get correction that is perhaps as good or even slightly better using non-ED glass by making the scope a triplet!!! Thats right. Just using a third lens gives the designer far more power in correcting chromatic aberration.
Next to this, a triplet using FPL 51 would be almost perfect. It would be almost impossible to see any chromatic defocus even on the brightest targets when used visually. For visual use at 4", an FPL51 triplet is sensibly perfect.
Ah, but what if you want to image. The camera lens is far more sensitive to red and blue light, so now, the super-perfection that imagers demand might mean that for some, the price of the FPL 53 triplet makes sense.
Hope this helps.