I see you are using a standard kart spindle now. That is the right move.
Run those in the lowest Ackerman setting possible (will explain). Generally that means the holes furthest from the center of the chassis. With your rack setup you will have to measure this though. Just measure the width of a point on the wheels when they are at zero toe, then turn to lock, and measure the difference (front to back on the wheel). This will tell you how much toe you gain or lose across your degree of lock.
With three wheels you have zero rear roll stiffness. The only way you can generate that is via the front. Your spindles have significant scrub radius, meaning any castor (spindle bolt top closer to rear axle) while create significant lowering of the outside front tire and a raising of inside front bolt. This helps center the steering going straight, but will create a chassis that wants to roll outward with (especially) three wheels. Ackerman would only make this worse as the inside front will turn greater and lower further. Additionally, a three wheel vehicle tends to turn very quickly as a result of zero differential action (needed). Ackerman increases turn in, meaning if you have a bunch of ackerman you are looking at a really unstable vehicle at turn in.