is it easy to transfer internally from back office to front Office in（jp/ms/gs)?

No

its usually harder because of corporate politics. most of the BO -> FO moves I see involve changing banks

Considering how incredibly competitive it is to get a FO job at a top 3 BB, do you really think it would be "easy" to transfer from BO/MO to FO at a top bank?  Surprised that someone would phrase a question this way.  

BO has pretty much zero transferable skills to FO. You’d have a better chance laterally from an IB boutique to FO than moving from BO to FO. You see a lot of MM IB move to BB IB. Students will take BO roles at GS/JPM/MS because of the brand name when in reality IB at a lower BB or MM far surpasses BO.

Hi- contrary to what some others have said, some of my friends have been having success. I have a few friends at banks such as CS/RBC London who work in middle office risk etc. and have had a couple opportunities to interview for front office positions internally. They have not been there all that long either so in my opinion it is a good way through the door and after that, it's up to you to network and wait for your chance to come up.

Like others have mentioned, it’s extremely difficult to move from BO -> FO at a BB. I previously worked at a BB in BO and found that it was near impossible to make the switch. I ended up going to a MM and joined a TMT IB team instead. Making the switch from BO -> IB is not easy either. Don’t expect your interviews to be easy if you interview at other banks. If anything, they will grill you more on the technicals since you are coming from a non-traditional background (at least from my experience).