What size 3K video? 2280 X 1080, 3072 X 1307? There does not seem to be a standard. What format are you trying to edit? What is your comp size? What is going on in the comp? What is the source of the original footage? Editing should be done in Premiere Pro, not AE. AE is for creating shots or short sequences, it is not a Video Editor at all.
Is the problem with Ram Preview? If so, then you are probably either trying to use MP processing when you should not, have not properly allocated your ram, or maybe your disk cache is set up incorrectly. It's hard to say.
If it is a problem with Rendering with the Output module then you may have to purge memory every few frames. You set that up by going to preferences and then before clicking General, hold down the shift key. At the bottom of the list you will see Secret. Go to that page and purge every few frames.
If none of these suggestions help please give us details.
3072x3072, or even i have videos at 4096x4096 :/ they are for Fulldome.
About the RAM, i don't know, and ill search for the option you said, thanks.
If your problem is just rendering, with frame sizes that large I would open up the secret preferences and purge every frame.
The other important thing is to make absolutely certain that the codec you are using to render your deliverables supports those frame sizes. H.264 can be very very picky about frame sizes.