Pontoon Boat Waterline

I want to calculate the power needed to propel a trimaran pontoon boat (low speed, about 10 -15 Knots max). I'm starting with calculating speed using the equation:

Theoretical speed (Knots) = 1.3 X Sq Rt of waterline length (Feet)

My question goes to determining waterline length. When considering a pontoon boat as a displacement hull, is the total waterline length the sum of three waterlines if three pontoons are used. Thus, the waterline length of a boat with three pontoons, say each with a 10 foot waterline would actually be 30 feet.

It seems logical that I will have three hulls in the water, but is this a correct way to determine waterline length?

No
From your illustration, there are three 10ft boats with 3 bows, sterns and wakes.

Calculate the power needed to propell one tube
Then multiple by 3

Then add a bit of power to counteract the extra drag because of wake turbulence interaction.

The formula you have quoted is a rough approximation for the maximum speed of a displacement hull with an 'average' Length / Beam (L/B) ratio (where an average L/B would be around 3 usually).

Pontoon boats have long narrow hulls - the L/B ratio might be 10 or more, so this formula does not really apply to them.
How big is the boat you want to calculate the power required for?
Can you post any photos of this boat?

Pontoon boats also drag a submerged transom which adds to the hull resistance.