1. "скажите"
"сказать" is possible here, but... I whould say, if you read such a phrase, you think by default that is has a little bit "stern" or "command" tone. Without knowing context or hearing voice tone I whould think so.

2. "будьте любезны" is a little bit "old-fashioned", a very little bit. politness is the same.

3. you can. 'сказать' almost doesn't fit here.