The MikroTik switch configuration methodology in RouterOS is second in frustration quotient only to an HP switch I dealt with 10 years ago. MikroTik could at least make a wizard in WinBox which could allow you to select tagged and untagged radio buttons per defined VLAN and generate the arcane code on the backend automagically.
I've not tried VLANs on an RB750, so I'm not sure which switch chip it has.
I think some of the lesser switch chips, i.e. the ones *not* used in the CRS devices, may not have the ability to switch tagged to untagged ports themselves. I tried really hard with a CCR1009's four port switch and failed last weekend. I finally just created VLAN interfaces and bridged them to the ports I wanted untagged. The CCR had the horsepower so I wasn't too worried about it. I also failed to accomplish what looked like it should be possible with the switch chip on an RB493G about a year ago. But I didn't dig in too hard on that one.
In your case, I would probably create ether2vlan10, ether2vlan11, and ether2vlan12 on the RB750. I would then create bridges named VLAN10 and any other VLANs which need to be on multiple interfaces. I would add ether2vlan10 as a port in the VLAN10 bridge. I would add ether3 as a port in the VLAN10 bridge. According to the layout specified in your original post, that would have you all set. I suspect your specification is incomplete or has at least one typographical or logical issue since "vlan9" is specified only on ether4 of the RB750 and is untagged there. In a router, it's not really a port based VLAN. It's just another, probably routed, interface.