Triskates usually have a similar, if not the same wheelbase as a regular skate
Advantages
Having larger wheels enable triskates to have a higher top speed. This is due to the wheels covering a greater distance for each revolution of the wheel due to its larger diameter.
They also allow triskates to bank at a greater angle as you can bend the skates more without touching the ground
Additionally, larger wheels also enable triskates to more easily roll over small debris, resulting in a smoother ride
Since triskates only have 3 wheels, there is one less point of contact with the ground which makes them better at retaining speed due to having less friction with the ground
Having 3 wheels also means you have 2 fewer wheels to maintain, one for each skate, so you can spend more time skating and less time maintaining your skates
Triskates are easier to slide with due to them allowing for a greater banking angle and having less friction
Triskates are also lighter than regular skates in spite of their larger wheels because they lose the weight of 1 spacer, 1 axle, 1 wheel and 2 bearings when losing 1 wheel on each skate. This allows triskates to have a greater top speed than regular skates.
Disadvantages
The larger wheels make it more difficult to accelerate as more torque is required to turn the wheels. You can reason about it using torque = force x radius. When the radius of the wheel increases, the torque required to turn the wheels increases.
Having 3 wheels also causes triskates to be more difficult to accelerate. The reason is that skaters push their skates against the ground to gain speed, which means that friction is the primary force for the acceleration of the skate. A triskate having 3 wheels has fewer points of contact with the ground, which would mean less friction and hence less acceleration as force = mass x acceleration. This increases the difficulty of accelerating on a triskate.
The larger wheels raise your centre of gravity which makes triskates more unstable than regular skates
Larger wheels also tend to be more expensive than smaller ones, which makes triskates pricier than regular skates
The space between the wheels in a triskate is also larger than on a regular skate, which makes it easier for triskates to get stuck when rolling down kerbs, bumps, and stairs
The centre of a triskate is in the middle of a wheel, rather than being in a gap between two wheels, which makes it more unstable when going down kerbs, bumps, and stairs