I've said it a 1000 times.!!! . . . Build in adjustabilty and between this, shimming and patience you will get the accuracy you require.!!
Most don't even know the accuracy they require and most are aiming far beyond there capabiltys to achive what they think the need.! When the reality is they don't need anywhere near the kind of accuracy they think they do.!! . . . . . For instance whats the point building a machine to 0.01mm accuracy for cutting woods that only need 0.1mm accuracy at best.?

Boyan if all you have is 0.15mm error over 3m length then I'd say your got it about as good as it gets with a single action. Now it's time to either shim or fill the low spot and scrape level. The important bit is that you know the 2 rails are on the same plane any surface discrepencys can be filled or shimmed out.
Look for epoxy putty and fill the low spot then scrape or sand it level checking with your straight edge.

IMO your expecting too much from epoxy over this length.!!