Would it be advisable to hire a car.
Yes, hire a car. While you could in theory manage a trip to those places (I've been to all of them at least a couple of times) with public transport and taxis (and a bit of walking), it would be a complete and utter pain in the arse even if you did speak Polish.
There's no problem at all with getting a satnav in English (the best program to go for is called Automapa). I'd be amazed if car hire companies don't have them available to rent out but you could also just buy one for about £60 (might well be cheaper than renting it from the car hire company).
BTW: if I was planning a trip to see them and had minimal time, I'd do it as follows: Warsaw to Treblinka (90 minute drive), 2.5 to 3 hours at Treblinka (assuming you want to see both camps, there's a 2km walk between the two, do not believe your sat nav if it tells you you can drive between the two), Treblinka to Wlodawa (3 hours), overnight in Wlodawa. Wlodawa to Sobibor (20 minutes) (note: Sobibor death camp is not in Sobibor village, it is actually close to a tiny village named Zlobek Maly, take the road to there from Wlodawa and keep going for about 4 km), hour at Sobibor, Sobibor to Belzec (2 hours), hour and a half to two hours at Belzec (the new museum is worth taking time over), Belzec to Lublin (two hours), overnight in Lublin. Lublin hotel to Majdanek (15 minutes), two hours at Majdanek, Majdanek to Warsaw (2.5 hours).
But, you need to understand that driving on Polish roads, especially the roads you'll need to use for that trip, is a lot more tiring than driving on motorways or even British A roads. Those are basically B roads almost all the way. So you might want to add an extra night and stay in Zamosc (which is a stunning little city anyway). If you let me know how much time you have, I can recommend where to overnight and what things on the way to stop and look at, for example: if you've read Ordinary Men, you're going to want to detour to Jozefow; you'll be going right through Izbica, which is the site of the camp Karski mistook for Belzec; Lublin is where Aktion Reinhard was planned and directed from, the HQ building is still there, so is Globocnik's house (now lived in by the son of the man who was thrown out to make way for Globocnik) and so is the former SS brothel.
Take train to MALKINIA which is some 4 km away from the museum. The trip takes usually less then 2 hrs.
Yes, but the bridge over the Bug by Malkinia has been closed since at least 2008, so there's a need to detour about 60km and that taxi fare (even without the waiting time charge) will cost more than hiring a car for the day.