I apologize but I am not following you entirely. If you stack the 3750, you must use the stacking port and use the stackwise cable.
You said: I have 1 stack of six 3750 switches with a 10.50.3.10 ip address
>> This would mean you stacked them using the stackwise cable and all these six switches are seen as one single device.
What do you mean by you have int 6/0/19-21 assigned? Assigned them what?
You said: I have a second ip scheme with one switch with an ip of 10.50.3.11
>> Sounds like you have another stack? Because the device will complaint if you address two different interfaces in teh same switch/router to the same subnet (10.50.3.10 and 10.50.3.11), unless these addresses are masked as host but I doubt that.