Rental expectations for a beach house split into a tri-plex?

[Palmer Thomas]

I'm trying to run the numbers for a beach house in South Carolina and am having trouble figuring out the expected income. The house is 7 bed 4 bath in the Myrtle Beach area, but the house has been split up into a triplex consisting of a 3/2 top floor and two 2/1s beneath that. When I use the STR estimate tool here on BP I get the units bringing in somewhere around $45k each per year. I've also discussed the house with a property manager in the area and they estimate that the entire house would bring in around $75k a year once it acquired an established rental clientele. The two numbers ($135K from str tool and $75 from prop mgmt) are really far off.

Does anyone have any experience with a house like this?  I was thinking that the units would be full in the summer and that they would also do much better in the off season than a regular 7 bed house because in the off season there are many more people looking for smaller units and not entire families coming for vacation.