3 white wires and 1 black wire in ceiling fixture
The black wire is the hot wire, the white wires are neutral and ground is ground. You need to connect the black wire of your light fixture to the black ceiling wire, the white wire fixture to the 3 white wires, and the ground wire to the ground wire. You have to maintain the connection of the 3 existing white wires. You may need a larger wire nut to connect your fixture white wire to the existing 3 white wires. It may be better to use a jumper wire and 2 wire nuts to connect the 4 white wires together.