Leave the breaker off and don't keep resetting it. The tripping indicates a fault, likely a short circuit. Because the original connections have been disturbed and the breaker now trips, have a qualified electrician trace the ceiling and switch wiring and check for damage before reconnecting the fixture. Before anyone touches the wiring, they must shut off the power and verify with a suitable tester that no voltage is present. [ESFI safety guidance](https://www.esfi.org/test-before-you-touch)

Two black and two white ceiling wires don't tell us enough to determine the connections safely. In older switch-loop wiring, a white wire can carry hot power instead of being neutral, so connecting wires by color can create a short.

One possible arrangement has a power-supply cable and another cable carrying power to and from the switches. One connection at the ceiling feeds the switches, and a separate wire returns switched power to the lamp. Your description doesn't identify which wire does what.

Once the wires have been positively identified, the new fixture's connections are:

| Fixture wire | Connects to |
|---|---|
| Black | Verified switched hot from the switches |
| White | Verified neutral |
| Green or bare | Verified equipment ground, with required fixture and box bonding |

The existing 3-way switches can still control a fixture with those three leads. Any splice supplying power to the switches must also be preserved.

The old fixture's "little metal tab" doesn't tell us how to group the ceiling wires. Don't try different wire combinations to see which works.