Actually, all three are grammatical and mean the same thing. There is nothing wrong with using the present tense in English to talk about the future in some instances:
It is Children's Day next Tuesday.
Usually, we would word it this way, however, if we were talking about the future in this manner:
Children's Day is next Tuesday.
We use the simple present with the verb "to be" in this manner to talk about things usually near in time:
Christmas is next month.
We also use it when we want to tell someone that it is not near in time:
Christmas is not for another eleven months.
Although we could say it both of these ways as well:
Christmas is not going to be for another eleven months.
Christmas won't be for another eleven months.
We can use "will" and "be going to" pretty much interchangeably, but "be going to" is more often used for events that are in the future, but near the present time:
It's going to rain later today.
But, in this instance below, both "will" and "be going to" can be used:
Christmas will be here soon enough.
Christmas is going to be here soon enough.
Regarding your three examples above, I would conclude that all three can be used to mean the same thing.