Since your a professional fraternity, I'll tell you what the ones on my campus did.
First night is a general info night. Have a presentation on what Phi Beta is all about. History, ieals, etc.
Second night is pretty much a get to know ya type of night. Chit chat w/ refreshments.
Any other nights should be an actvity that represents your group. ex the business fraternity would have a resume night
Your fraternity might do something like have the members show off their talents or ya'll may decide to attend a local concert or play. After a few nights like that you'll have another chit chat night and maybe some sort of invite only event.
After that you'd give out bids and have some type of celebration.
Professional fraternities at my school have rush drag on over a couple weeks. I'm not really sure if it's to elimnate looky loos or to make sure everyone has a chance to meet them. Either way, make sure the length of rush is satisfactory to most of your members. If they think 3 nights is too short do more. If they think 2 weeks is too long, do less.
Good Luck
Oh and never hesitate to ask for advice from your alumni and nationals!