Disconnect your 3.3V display from the Uno, Mega.
Upload any program to your 5V Uno, Mega.
Disconnect Uno. Connect 3.3V display. Power the Uno with 3.3V not USB.
.
Or run the Pro Mini at 3.3V with an external 3.3V Serial dongle.
.
Or buy thirteen level converter chips.
.
It is much easier and cheaper to buy a 3.3V Zero, Due clone. Or a switchable Uno clone like me.
.
Please say which 3.3V strategy you have chosen. Or post a photo.
The display may already be damaged by 5V.
David.