ARC PH 3 SE vs PH 7

Would agree with Jimf.
If you can find a used Audio Research PH-7. It may take awhile here, as they come up only every few months at best. You would need to watch the ads every day, like several quick peeks and be ready to buy instantly.
Wish you luck.
(I own an ARC PH-2 an underrated gem if used with great power conditioning, and an ARC Sp-15, with phono. My first phono is a Bryston BP 1.5)

Without knowing the rest of your system, my answer to your question, there is no comparison between the ARC PH-3se and the PH-7. The PH-7 is in another league.
The price of the PH-3se $2500 the price of the PH-7 $6000 that alone should tell you something!
I too have had the PH-3se. It is a good phono preamp, I replaced mine with the PH-5 which is better but not by a whole lot. The one thing that makes it better is that you can change cartridge load settings with the push of a button whereas with the PH-3se you have to go inside an change the resistors to switch load settings which can be a hassle particularly if you are not good with a sodering gun.
I would have the PH-7 right now if I had the bucks (and will at some point) it is considerbly better than the PH-5.
If you can afford the PH-7 it is a no brainer if not I would go with the PH-5 but as has already been mentioned, listening for yourself is always the best policy.

Thanks for your replies-the are unanimous. I don't have the bucks to upgrade at this time, but now know what to aspire to.