What tense is better to use ?

I’m sending you the sheets tomorrow at 5pm. (As it was planned before saying.)

I will send you the sheets tomorrow at 5pm.

Or there is another alternative in my mind

The sheets will be sent tomorrow at 5pm


These are all valid and correct ways of expressing this fact.

The simplest way to to state a future fact is "I'll send you...". It is the best way to express a promise to send. The third example is the same idea, but expressed with the passive. You could use the third way if the person who does the sending isn't important or is unknown. The first way indicates a future action that will occur as a result of what you are doing/planning right now.

Probably just use the "I'll send" sentence.