I am trying to install a new light fixture where I had a ceiling fan. The problem I have is that the light fixture I am using has a "fast attach receptacle". The box where the old fixture was has a bare ground wire, a black wire, and 3 white wires twisted together. The white wires twisted together do not fit into the opening on the fast attach female receptacle.

What could be a solution for this?

Use a wirenut large enough for these three wires plus an additional short pigtail to attach to the connector.

Usually red wirenuts will fit.