Raising rent when buying triplex

[Alexander Ransom]

So I found a triplex for sale in the Madison area that has three units and each one has been occupied for 15 years. It is clear to me that the rent has not been raised in a very long time. The gross rent is currently $1060/month. I feel that the rent should be raised significantly to gross at about $1700-$1800. This deal would only make sense if the rent were to increase. Any advice on how to manage this situation? Do we raise it slowly each year? Would we let them know before we buy the property? Any help/advice would be appreciated! 

Alex