Try graphite lube. You might need to get the current grease out of the mechanism though?
Put the padlocks in some old socks or something over night. Heat the key up with a lighter before you insert it?
Even with the key in, the internals were frozen solid. Might try socks and a different grease. Not pissing about with lighters though, if it comes to that I'll get a fucking blowtorch.
There's a fox (not the speccy kind) which keeps coming into my garden and removing the sock/rag that covers the bottom lock. Result, still having to urinate on my shed door at 6am to open the thing but only the bottom lock as the top two are OK.
I should add that I dried the out in the oven before greasing them up.