They are great little speakers. I'm saving up to buy the R5's only because it seems to match my listening height. Highly recommended.
In my experience I have found no appreciable difference when sitting on the floor (to get my ears to tweeter level which is very low on the R3's) or sitting in my armchair where my ears are about a foot higher up than the tweeters. Everything still seems to come from the right directions.
This is all at about 8ft distance with the R3's seperated by 6ft and facing square forward (no 'toe in') and with the side mounted speakers facing each other. Concrete floor, thick carpets, and just over a foot from a wall. They play across the shorter dimension of a 13ft x 21ft room with 8.5ft ceilings.
My armchair is not 'on-axis' and yet this configuration sounds fine. I have no need to hunt for 'sweet spots' like with my previous speakers. (MA BR1's on Atacama Nexus 6 stands) I would only buy the R5's to get an upgrade to the whole sound and not just because of the dimensions of where the tweeter is in relation to your ears.
If you have heard both and found your experience is different, in your room (and tweeter position IS a problem for you) then of course my experience is not valid and I defer to your circumstances. I know there are 100's of variables when it comes to room/speaker/furnishings dynamics but I have not found the R3 tweeters being so low as to be any problem at all, even with them being about 21" from a thick carpet.
Rega P2
Ortofon 2M Red
Rega Fono-Mini
Arcam Solo-Mini
Rega R3 loudspeakers
Chord interconnect and speaker cables
Isotek Premium & Tacima (mains cable and block)