In his section on Platy's he states that the Sunset or Varigated Platy should be kept between 68-75 degrees. Other Platy's at 75-79degrees.

Do you think they have ich, or bacterial fin rot?
Could the first one have been a victim of foul play? Do you have a fin nipper in the tank?
If you suspect fin rot, I'd suggest getting medication for that, not just a preventative at this point.

Follow the directions on the meds., see if it says anything about increased temp helping. I know it helps with ich, but I'm not sure if it helps with fin rot.

As far as your guppy that died, they have a life expectancy of only about a year. I know that's short, so it could have been old age.

For the gup that has the bent spine, I really struggled with my decision, you could probably tell.  But I felt that it would be more cruel to allow them to continue to suffer, then to put them out of their misery quickly.

Also, if I kept them, I could never allow them to breed, (life without hanky-panky? ;-) AHHHHH!! CRUEL! ) so where would I keep them?  And since I had three what if they were males and females?

I could just see myself trying to explain to someone, "Oh, those two tanks in the corner? Nevermind those, they're just my bent, twisted, mutant guppies."

Anyway, I hope this helps, I'm really not trying to make light of it. I know it took me some time to get my heart and my guts to do it.

But it WAS VERY QUICK!!! They were gone before the water reached the top of the measuring cup.