I'm Weak... Very Weak

Before i start getting into my SS build i wanted to see if i could handle it fitness wise, the bike would have been for my commute to work, but sadly after borrowing a mates bike for a couple of days i found that i had seriously under estimated the effort it took/takes to climb the what i thought were slopes and not hills on my way back home.

The next best thing for me i think until i'm fitter is a 3 speed, so question is can you build a 3 speed hub into any rim as long as it has 36 holes (which the 3 speed hub has) or are you limited to your rim selection.

I started 3 years ago on a massive 27 gear hybrid, after a while I thought' I dont use most of these gears' then went down to a 12 gear racer, thought the same thing again and went down to 6 speed, only really used 3 of them . so went to SS, of course it then follows you go to fixed. Been riding that for a month, each day am getting a bit stronger. It takes time but the more you ride the easier it gets. If I can do it at 45 I'm sure you can too.