3 season room vent and insulation questions
When you say, “metal roofing”, you mean metal roofing on moisture barrier on wood sheathing, right? (It’s not just metal roofing on joists, right?)

When someone tells me that they are building a three season space, I think it’s for use most of the time, but not the extreme conditions. Yes, having insulation in the walls and ceiling is preferred for the quality of the environment in the space. The more insulation, the more seasons the space can be used. 

No, just a ridge vent is not enough. The Code requires a 1” air vent space between the insulation and the roof sheathing AND cross-ventilation is required. The Code requires 1/150th the square feet of the attic space to be vents, unless 40% - 50% of a he area is within 3’ of the ridge. (Your table vents would qualify.) Then you just need 1/300th square feet of attic space for venting. 

In greenhouses we’ll often use just fans to exhaust air to control temperature, and no insulation. But that can be noisy.