How highly would Mayweather be rated if he accomplishes the following feat?

[Kel]

1. Defeats Mosely

2. Defeats Margarito

3. Defeats Cotto

All in consecutive order then retires. How high would he be ranked? Can he be a top 5 all time great?

Depends on a number of variables...
How convincingly does he defeat them?

How impressive do those fighters look before/after they fight Floyd?
For example, if Cotto barely scrapes by Judah, a Floyd victory over Cotto doesn't look as impressive as it would if Cotto destroys Judah in a couple rounds.

My general stance is that if Floyd cleans out 147 over the next 3 or 4 fights, he'll be at least Top 20 all-time.

I agree with that mate. But can you see him cleaning out the 147 division?