I currently have STH-ETH-200 SmartThings hub and want to bridge to a Aqara M3 matter hub. Can I do this ?
No, no SmartThings hubs act as “matter bridges“. At the time of this posting Samsung only supports matter one way in, not out.
That said, the aqara M3 hub is both a matter controller and a matter bridge, so you can connect it to your SmartThings Account by bridging out from the M3, if that’s of any use to you. But it may not be what you were looking for.