I've a Btwin Rockrider 5.0 which comes with SRAM MRX Twist Shifters (21 spd 3x7). But lately they have been giving me problems in shifting, I've had bad experience with twist shifters earlier also( Shimano Revo), thus I want to change the shifters to trigger shift. Can anyone suggest which one's are compatible with SRAM MRX (21 spd 3x7) shifters.
The MRX twist shifters are 3x7 Shimano-compatible shifters, so you can use any 3x7 Shimano shifter set from the mountain-type group.
Unfortunately, the only trigger shifters which are 3x7 Shimano compatible still made are from the Tourney line, such as the Shimano Tourney ST-EF40, which are of rather low quality. You may be able to find an old Shimano Acera or Alivio or Altus 3x7 shifter set if you look on Ebay or something.
I had a 3x7 drivetrain on my commuter, and ended up swapping the rear freewheel (yes, freewheel and not a cassette) with a 8 speed freewheel, and getting some Shimano Alivio 3x8 shifters which were easy enough to find and of good quality. I wasn't able to get the Tourney shifters to give good shifting performance, and I don't like twist shifters (for the 3x7 drive trains, I suspect SRAM's Shimano-compatible twist shifters are the best of the indexed shifters which are still on the market in terms of reliability; they're all cheap pieces of junk these days).
Another alternative is to use a friction shifter. Very cheap to buy, though you just have to get used to shifting into the right position by feel. This is what I'd likely have done if I didn't upgrade to 8 speed.