I'm staying here for three days > your stay here is intended for 3 days
"I'm going to stay here for the next three days (as from today."> You will stay 3 days here , and today is the 1st day. ( from now = the above is the better form). But there is no need to mention that since it is already clear without it.
From now onwards , I will ....
As from now , I will ....
"I've been staying here for the past three days."> You have (already) stayed the last 3 days here. ( you are still staying there > doing something from a point in the past and continuing to do so till now> present perfect continous tense) . If you stay another night , you will then say, " I have been staying here for the last 4 days."
"I'm in the middle of my stay here, but anyway, my stay here is three days in total."
> You are halfway into your stay here , and you have 1.5 days to go before you leave.