The MCS rate is per access point radio, therefore having 3 x 1250s or 3 x2125s makes no difference.
The 2125 only has 100Mbps ports therefore this would be the maximum throughput so channel bonding is a mute point. MCS 15 gives rates to 150Mbps or raw data just over 100Mbps. MCS 11 would be the highest value that could be achieved with a 2125 as the 100Mbps is the bottleneck.
MCS also depends on the guard interval chosen either 800 or 400ns. The lower guard interval gives marginally better throughput.
I assume you could conect to the access [point at MCS15 but would only see 100Mbps through, I guess I will have to experiment