Creating a second SSID on the same AP(s) will make the issue slightly worse as more ait time will be consumed with beaconing.
Using different brands of APs is likely to cause issues. You might find air marshall is seeing the non-meraki AP as a rouge and is actively disconnecting users from it. It could be that users are trying to roam and the roaming is not working well between the two different brands.
I would be tempted to replace all three with a pair of MR42's, or if everyone is in one area perhaps an MR53.