tl - mr3020 v3 support in trunk added in September 2018. Why is there no firmware in release 18.06.2?
The openwrt-18.06 branch was branched off master in late june/ early july, which is too early for that device (and larger changes that don't fix bugs are generally not considered fir backporting, this more or less precludes support for new devices). If support is in master/ snapshots, it will be part of the next (major) release, 19.01.0 - but neither 18.06.2, nor 18.06.3.