In 10.3/10.4/10.5 you can create a L3 LAG on the same CX switch
but not a L3 VSX LAG (MCLAG). This is not yet supported.
For the above simple topology, I would suggest to use OSPF routing
on each point-to-point L3 link. Typical convergence time in case of link failure should be less or equal to one second with default OSPF timer.
If this is not fast enough, you can tune OSPF spf-throttling timers.
The other alternative is to use VSX LAG between the VSX pairs, with transit VLAN (and associated SVI). This improves the convergence time down to few hundreds of milliseconds, if needed.