If these are the coordinates of A, B:
A = (Ax, Ay)
B = (Bx, By)
Then the vector from A to B is given by:
vector AB = (Bx-Ax, By-Ay) = (BAx, BAy)
And the unit vector (the vector of length 1) which points in the same direction is given by:
(BAx, BAy)
unit vector AB = ------------------, where length = sqrt(BAx^2 + BAy^2)
length
Now, the unit vector which is perpendicular to AB is given by:
(-BAy, BAx)
unit vector perpendicular to AB = -------------
length
There are two possible unit vectors perpendicular to AB. The one shown above is what you'd obtain by rotating the unit vector AB counterclockwise by 90 degrees.
Given the above calculations, here are the desired coordinates:
coordinate at t1 = (Bx, By) + t1 * (unit vector perpendicular to AB)
coordinate at t2 = (Bx, By) + t2 * (unit vector perpendicular to AB)
coordinate at t3 = (Bx, By) - t3 * (unit vector perpendicular to AB)
To be explicit,
(Bx + t1*(-By+Ay), By + t1*(Bx-Ax))
coordinate at t1 = -------------------------------------
sqrt((Bx-Ax)^2 + (By-Ay)^2)
The others formulas are very similar.