What to do to enable communication of a VM with a physical server (also connected to 10GbE network) where data transfer must hit at least 3GbE (some 300MB/s), is it even possible or not?

It sure is possible. I myself rand throughput tests with iperf and Linux VMs (using vmxnet3) where I easily hit 9Gbit/s bewteen different hosts on a physcial 10Gbe network infrastructure. Not sure I understand your "question" there, but you don't need to do anything special for this, as long as you have the capable physical infrastructure in place.

You actually don't even necessarily need vmxnet3, but it's highly recommended since it saves a lot of CPU cycles. That's because real physically imposed wire-signaling limitations do not apply in a virtualized environment. OSes don't artificially limit traffic to match the agreed on line speed unless it would be physically unavoidable.