Thanks to both Oxford Diecast and Golden Valley! They suggested to strongly clean the wheels. I cleaned them with abrasive toothpaste and a brush. That helped a lot - the loco ran on the rails.... But ! Still badly ran though switches since the 6 wheels have bad contact. We can see that the 3d wheelpair is sprung, but that is not enough. At least 2 wheelpairs must be sprung to provide proper contact on switches.
I have installed ESU PowerPack Mini and that solved the issue fully. I can conclude that power pack is "must have" for 3 axles locos. Flywheel is not comparable with powerpack in effectiveness. Now my Janus can shunt on a very complicated layout with a lot of switches. I even have the reversing loop on my layout with 11 cm (4.5 in) isolated (non-electrified) rail section - and Janus easily go through it.
Also, as an engineer, I would suggest to gear the wheels together, while the stick should be just cosmetic.