Financing more single family rental homes.

[Benjamin Fye]

Hi, very new to this site and have been listening to the podcasts (which I feel are a wealth of knowledge).  I will give a little bit of a snap shot of what I have going on.  I live in MN in a city of roughly 90K people.  I currently have my primary residence with approx. 100k in equity.  I also have three other single family rentals.  These are all close to two major colleges and the houses are rented to college students.  I have put 20% down on the three rentals and financed them with 30 year fixed mortgages (3.75-4.5APR).  I have 318K currently financed on the three properties and they are worth approx. 510k.  I profit a little over 3k a month after all the bills for the three houses are paid.  I also have a career that I make 75k-85k per year.  I would like to secure another 2-3 houses this next year.  My question is should I refinance my rentals for the down payment on more investment properties?  Pull out line of credits on them?  Or go a different route?  Thanks in advance for any advice and I apologize if this isn't posted in the right spot.

Thanks, Ben