At least 20 Amps if not more (probably nearer 30 Amps.) for a 3.0mm2 conductor buried in cool soil in the U.K.
Table 4D4A states that 70 degree multicore S.W.A. with copper conductors, ref. method C (clipped direct) size 2.5mm2 (ground ambient temp. 20 deg. C.) can carry 28 Amps. So I reckon that your 3.0mm2 will carry 30 Amps comfortably.
take the 3 phase rating, as the rating of 3 cores. - in this case 18 or 21A clipped direct per core, depending on copper temp of 70 or 90C.
Then take say you can take the same heat load per core - so talking the 70C figure, as we do not know, in 3 phase it can take 3 lots of 18A (54A) for the total cable, but power ~ square of current, so dividing same heat load over 4 cores instead of 3, >>3/4 power is sqrrt 75% = 86% of current, so .86* 54 =46A then divide by 4 for current per per core or more usefully 23A per pair of cores for the same heat load as the 3 phase full load.
I am inclined to agree it could probably be pushed a bit as the 3 phase rating does not really assume a completely cold neutral. If the run is short watch the end termination quality to make sure the current sharing is good.
