What's the deal with current MM carts boys? I only ask because I remember a couple of old Ortofon's of mine - the capacitance they saw was critical to the HF performance (Nopiano you may have had a cap. trim switch on the front of your HK amp like I did)
Wasn't it the 'Variable magnetic shunt' principle that made Ortofon's so susceptable to the capacitive loading?
I only say this because the capacitance of coax varies significantly - with the wrong type, 3 metres could add more capacitance than that trim switch used to (and you could easily hear the difference with that)
I agree with Al that ideally the OP would have the TT close to amp and that a phono preamp, though a bit of an overkill, would be a good second choice as a booster. (OP note the output of any external phono preamp would go into a line input on the Onkyo, not its phono input)
If I were you OP, I'd go for high quality coax, preferably with the lowest capacitance (quoted per metre) then compare the sound of the TT with the current and extension leads. We all think it better not to put the low output of a cartridge through 3m of cable, but if you can't hear a difference, no problem.
Alternatively, you might want to get a much better preamp than the one in the Onkyo and use it as a booster so that the majority of the 3 metres is done at line level. You'd also have then have the bonus of not needing to consider a built-in phono preamp with any future amp upgrade.