Since Venice has 25 more theme parks than Jamestown, there are 20+25 = <<20+25=45>>45 theme parks in Venice.
Together, Venice and Jamestown have 45+20 = <<45+20=65>>65 theme parks
With 50 more theme parks than Jamestown, Marina Del Ray has 50+20 = <<50+20=70>>70 theme parks.
The total number of theme parks in the three towns is 70+65 = <<70+65=135>>135
#### 135