Jim,
Assuming your original setup had a single panel, the 15A fuse would have been correct. Panels put out 17-18v DC and 100-140 watts, depending on type of panel. 140W would yield about 8 amps. Going to 3 panels, the current triples to about 24 amps so a 25A fuse is minimum. Keep in mind that panels will seldom reach their rated output which is probably why the 25A fuse was recommended rather than a 30A. If you are adding the 3 to the existing panel, the 25 a fuse would need to be increased
Steve