If a 15-minute nap during the day = 90 minutes during the night...

[*Laurelindo*]

...does this mean that two or three such naps is the same as 3-4 hours of sleep during the night?

It's said that napping during the day for 15-20 minutes will have the same effect as one and a half hour during the night, but is this cumulative?
In this case I assume that you actually fall asleep during the 15-minutes naps as well, so you have a chance to REM.

This is probably a rather far-fetched assumption, but it doesn't seem all that unlikely either.