I ran a search for 3 bedroom apartments or homes within your price range in Westminster and didn't find anything on the first floor or wheelchair accessible.
However, I did find an apartment complex that might be able to accomodate you, but the prices vary so I recommend that you follow up with them personally to check their availability. When looking more accessible housing, apartment complexes will probably be your best bet.
Here's where I found: http://www.trulia.com/rental-community/9000046590/Park-Place
Good luck with your search,