Yes, you can do that. Then it will allow that WLAN to preserve upto CoS =5 (DSCP EF). Depending on the wireless client traffic classification (WMM UP) different traffic type will get different type of CoS value within this WLAN. If you want all traffic to get CoS =5, I think (hv to confirm once I test it) you have to disable WMM under QoS section for this WLAN (note this would impact 802.11n data rates).
As long as you keep other two WLAN's profile to "Silver" all traffic will be capped to equivalant CoS of 2 max.
There is difference between WMM UP & AVVID 802.1p (CoS) values. Please refer this if you get confused
I have tested WMM disabled option for you. In that case all traffic will be marked with appropriate DSCP/CoS value as per QoS profile configured. In your case once you disable WMM on your platinum enable WLAN all traffic will get marked as EF.