Yes the long uneven front claws are a trait for males. The tail , it's more so the cloaca location, some tails alone may take more time to tell. And if you have a fat male would be hard on male to tell by tail alone. Claws and cloaca ! Unless your turtle is fanning at you !
You may of just been lucky on those other two in the past. Males for sure ? What size were they , important ? What type tank setup ? Amount of water alone ? How long you had them ? What happened to them ?
None of this is going to happen today you have time. Some have more time than others depending on there sexual maturity and aggression that shows up ! With four the odds are not with you ! Prevention is key for a long life.
Some beginners think a year in this hobby is a long time and some turtles have a short life when not taken care of . RES's in captivity well taken care of can live 40-50 years !
My original turtles would be in there mid 40"s today if Hurricane Katrina didn't take them. 7000 gallon in ground pond could not stand the 31' surge and when all settled down I had 9' of water where my house was 27 days later. Three were adult female RES's and two adult male RES's along with one common snapper. Also 10 Koi over 20" and some goldfish. Then a lot of mosquito fish. That size pond the males had there own places but did get into fights and had the scares to prove it but being such a large area did not kill. Too many stories in my time of indoor setups where it was to the death due to aggression and or dominance ! Seen my share on what happens from the center on what can happen being the only rescue site around this area . That was in the area , Katrina wiped it off the map. Twelve years later just starting to rebuild it and been asked to return but just to old now. Yes some may get along even for a year or two maybe more but those days do come. Then it's too late ! When people are told about it they just don't want to believe until that day comes for what ever reason.
All we swam away with were the cloths on our back and our lives. Life started over and I was already retired. Today can only afford an indoor pond and to keep cost down with one male RES , Piggley. Yes males are really less work due to only getting 9" and not having to worry about nesting areas. I use a 150 gallon stock tank for him. Cost 1/4 the price or less of just one regular glass tank same size. Only one's creativity limits on how the inside and outside looks. Piggley love's his home and I love Piggley also its make him feel like he in a nature pond. Piggley never seen a pond in nature with glass sides that more just for us to view. Here's a short video I use on my last update for the forum :
https://www.youtube.com/watch?v=KREuab7P8eAIt's Piggley's design , if I put something where he hates it he moves it , it's his home so I listen !