unsqueeze(-1) adds a new dimension after the last index, so the shape of t_b transforms into:
t_b.unsqueeze(-1).unsqueeze(-1)
 (3)  (3,1)        (3,1,1)      <- shapes

For comparison, if we wanted to add the new dimensions in “front” we would do:
t_b.unsqueeze(0).unsqueeze(0)
 (3)  (1,3)        (1,1,3)      <- shapes

Chaining two unsqeeze calls (or any Python method for that matter) together is equivalent to
t_b = t_b.unsqueeze(-1)
t_b = t_b.unsqueeze(-1)