Bike #1 - What are the dropouts, this could complicate going to single speed, although still doable, don't count on it being too cheap, you will probably need a new crank, chainring, single speed conversion kit for the cassette (assuming it's a freehub) & chain tensioner, as well as a new chain.

Bike #2, unless you can get a new fork, and front wheel (Dahon use 70mm hubs / 20" rims, from memory, which can be hard to source) it's scrap; hopefully you got this replaced (the value of) by claiming on the drivers insurance?

Bike #3 - is is physically possible to do this? Does the frame have the necessary mounting points, are the wheel rim brake compatible? If they are, are they painted, or do they have a a braking surface?