The difference between PP, FCP and DR?

I would stick with DR, if I were you. I use FCPX to edit small projects not needing much 'finishing' but, I use DR for anything else. DR can handle just about anything you throw at it. Not so sure with PP.

people do the difference, i used pp from 1996, resolve from 2009, fcpx from its born, after effects and other adobe procut from 1997. Certified user and trainer on many of that, and other about 3d, post, etc.

At today Resolve offer you integration of ingest, edit, color, vfx, audio, delivery that never Adobe tools offer you, be cause you must in out from software, use dinamic link that often give you errors and delays.
Offert a collaborative situation where you can in out from Resolve to many other editor and back, things that others wann't to do (to keep users locked on their suite).
If you ask me, i told you choose software that you think easy for your work, but test every single feature, be cause hidden problems are daily fight for software, and i can annoying you with a long list of problems for all of these softwares.

Did he atleast say why he had the idea that you could only edit in FCPX?

Seems pretty stupid to just demand without saying why.

Of the three I would say FCP is the weakest. And the PR / AE setup will loose ground once Resolve 15 is out of beta. It will incorporate Fusion in this release and the editing suite is powerful enough now for most people's use.

I have already left PR behind. It doesn't make sense anymore, and it's colorgrading tools are weak compared to DR.

So... if you are indeed going edit it from start to finish and export and deliver yourself, I see no sound reason that your client "demands" you use a specific tool. Your best tool is the one that makes you works the fastest without the fewest hickups.

I would politely tell my own client that while they may have used FCPX themselves, I will not.

This is just an assumption, but I imagine the client asked for FCPX (confirm X and not 7), because they know how to use FCP. If you had to send the client an xml or EDL there would be fewer hiccups from FCP to FCP. And that is true. I also assume you want to keep your client for future business so I would suggest you either use FCP for that client until you can convince them that you are more efficient, therefore saving them money, with Resolve. Aside from that I agree that you should use the tool that gets the job done for you. I use Resolve myself, but it is buggy and that is frustrating since it seems to always be in Beta. However, in the end, if you need an all in one program, Resolve is it.

Well first off, the industry standard is still AVID for feature films, both in Hollywood and elsewhere. Things might change, but currently it is what producers are familiar with, pipelines are constructed around and what makes everyone feel at ease at the top level.

As for the differences between the other programs, as I see it:

PREMIERE
PROS: Tons of features, integration with other Adobe programs, widely used, familiar to people who learned editing on AVID or old Final Cut.
CONS: Buggy and quirky, middling performance-to-hardware ratio, cluttered and constructed with a more-is-more mentality.

RESOLVE
PROS: Beautiful colour integration, good sound and vfx integration too, with rapid development by an ambitious developer. Plays well with most other programs, and smokes these other editors as a finishing tool. Free!
CONS: Rapid development means growing pains; bugs and design gaffes, very demanding on hardware (though that is improving), not widely used as an editor yet.

FINAL CUT PRO X
PROS: By far the biggest benefit to those who use it, is the altered paradigm – designed with a desire to change digital editing from the ground-up, from the organisation of data to the magnetic timeline, this is different. Those who like it LOVE it. It's also streamlined and sleek, and runs better than anything else on a Mac. Steady development and a mature and robust eco-system of plug-ins.
CONS: Strange blind-spots and missing features, sometimes things have been simplified to a fault (typical Apple). Requires expensive plug-ins to do basic things and bumpy integration into established vfx, color and especially audio pipelines. Hard adjustment for people raised on the previous paradigm.

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I myself have drunk the FCPX kool-aid, but use Resolve to finish my more high stakes one-man jobs. I have edited two feature films on FCPX now, and find the thought of going back to Premiere/AVID style editing simply depressing.