OSPF only supports equal cost multiplath routing. You could configure the cost on all links the same, this however would (statically) not send the correct proportion of traffic to the Ethernet link.

If the three 2 Mbps links were (MLPPP) bonded, then you could equaly split the traffic between it and the Ethernet link.

More complex approaches include: define 3 GRE tunnels across the Ethernet link, and equally route across them and the serial links. Or, if your equipment supports PfR with the PIRO feature, it could dynamically load balance across the different links. (You could also dynamically load balance using an earlier version of OER/PfR using statics which you might need to redistribute into OSPF).