A U turn is the sum of the previous and current speeds
So it means add them together.

My ship have engine capacity of 3 so:
1turn - move forward 3 hexes (speed 3)
2turn - speed lovered to zero
3 turn - U turn or (U3, 1U2, 1U1) (0+3, 0+3, 0+2)

Turn 3
U3: Previous speed 0+Crurrent speed 3=3
1U2: Previous speed 0+ Crurrent speed 1+2=3   So 0+3=3
1U1:Previous speed 0 + Current speed 1+1 =2

1turn - move forward 3 hexes (speed 3)
2turn - speed lovered to zero and U turn (3+0, 0U) 

Previous speed 3 + current speed 0 = 3

How about this

Turn one 5

Turn two 1U2

Pevious speed 5 Plus 1+2=3 So you would need 8 points of thrust to do this.

It depends on your speed in the previous game turn.

1 hexside turn = Thrust requirement is greater of last turn or this turn's speed
2 hexside turn = Thrust requirement is sum of last turn and this turn's speed
3 hexside turn ("U turn") = Same as 2 hexside turn

Assuming your speed last turn was 3, and your engine capacity is also 3, then you are allowed to make a 1 hexside turn and still move up to 3 hexes. You may also turn 2 or 3 hexsides, but in doing so your ship must come to a stop.

So yes, your calculations are correct, presuming your previous speed was 3.