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2) Let say if I stack 3 switches, middle switch should be Master. Uplink should be at Top and Bottom. Is best practice?
As far as I know, beyond having uplinks on different switch members to avoid single point of failure, there's not a best practice for which switch is stack master or which stack members host the uplinks. Common practice probably is to use top and bottom stack members for uplinks. In theory, might be some tiny benefit to having stack member on least busy stack member (such a stack member not hosting uplinks) but the bulk of traffic forwarding should be at the hardware level. If you have software traffic forwarding you likely have a bigger problem than will be addressed by which stack is the stack master.
3) Can we mixed 3750G and 3750X and what is result internal BW - fallback to 32Gps?
Yes and yes. Stack also falls back to Stackwise from Stackwise+, which might be just as bad a loss as losing half your stack ring bandwidth. (Mixed stack is also more difficult to IOS upgrade.)