Attach light fixture with 3.5" bolt holes to ceiling box with 2.75" bolt holes
I managed to attach the light fixture using the universal crossbar, but with some improvisation.

The light fixture included two combo truss bolts that were designed to screw into the junction box but this didn't work because of the incompatible sizes(3.5" and 2.75").

The solution was to  bolt the crossbar to the junction box and hang the combo truss bolts (to attach the fixture) from the top side of the crossbar, with the bolt heads resting on the top of the cross bar. Then I secured the fixture to the bolts using nuts.

Not perfect but the fixture is stable and secure.

Thanks for your comments guys!