What do these formulas mean? I know it's relating constant acceleration but why are there 3 different formulas?
The first tells us the velocity at time t if you know the answer initial velocity (i.e. at t = 0) and the value of the constant acceleration. It lets us answer Q's like "A car is accelerating at a constant rate of 4 m/s^2. If it started at a speed of 1 m/s, how fast is it going after 10 seconds?"
The second tells us what the particle's displacement is at time t relative to its starting point if we know its initial velocity and the value of the constant acceleration. E.g. If a ball is dropped, what distance has it fallen after 4 s? If a car is accelerating at a constant rate of 5 m/s^2, and started at a speed of 20 m/s, how far has it travelled after 10 seconds? We can answer Q's like this with this formula.
The third one is a useful formula relating the velocity to the displacement, if we know the initial velocity and the constant acceleration. It lets us answer Q's like "A car is accelerating from rest at a constant rate of 8 m/s^2. How fast is it going after it has travelled 100 m?" Or "A ball is dropped from a 100 m building. How fast is it travelling just before it hits the ground?" or "A car was travelling at a speed of 60 km/h. It needs to come to a halt in 50 m. What is the average deceleration required to do so?"