The water is always guaranteed to be level,
3" is a lot. I would suggest:
Step 1) inserting a 3" board under the edge of the spa that will establish the tub in a level position.
Step 2) using multiple strips of plywood, slide a strip of plywood under the spa from the elevated end where your 3" spacer is until the plywood stops after hitting the other end of the spa.
Step 3) add additional strips of plywood below the first one, pushing it in until it too stops, repeating this process until you have successfully filled the empty space to support the 3" elevated end.
Keep in mind, the shell wants to rest on a solid surface, and it has supports strategically located under the shell to equally disperse the weight over the square footage footprint. Placing plywood under the spa allows the base of the spa to remain in contact with the flat patio surface. Once a level is established, mark you boards to be cut flush to the primeter edge of the spa so they will disappear under the spa, but supporting the base. This will establish the level of the spa. If necessary, you can apply a piece of trim around the bottom edge to hide any rough edges.