Spinning three times/week will help you maintain somewhat of a base, but it depends upon how hard you push yourself. It sounds like you are pushing yourself if you feel that you are getting stronger. As has been pointed out on this forum many times, nothing improves climbing like, well, climbing. Dropping 10 or 15 lbs will certainly help a lot, too.
How much do you ride? I find if I don't ride 150 miles minimum per week, my (leftover winter) weight stays about the same, and I usually pick up about 5 lbs in the "off season" when I only ride what the weather will allow and attend spinning classes. After training for and doing a couple century rides I have a hard time keeping weight on! But, I'm one of those guys who can easily drop weight.
I guess the easy answer to dropping pounds is ride more, consume fewer calories. The climbing will come with practice and weight loss.