Is there an Interface Card that I can use with Mikrotik to be able to use our new OC-3 Line?

There is no such interface, which is supported by the RouterOS.

You can use a converter like RAD RIC-155GE, or a transceiver modules for switches like Axiom SFP-OC3-SR-AX.

What a shame. Good product but no interface cards. Oh well I guess it's off to Cisco

Do you have any clue what a Cisco 1-Port Channelized STM-1/OC-3 line card adapter cost? List Price: €32,400.00, and that is only for the expansion module for a Cisco router.

RAD RIC-155GE cost around $700, and Axiom SFP-OC3-SR-AX cost around $200.

This Axiom SFP Modules could work with a common ethernet switch with SFP ports ? Or it needs an ATM router that can translate ATM to ethernet?

Thanks