Your math is wrong (you left out two of the main engines):

1) SSME thrust is 418k lbf. The shuttle has three engines, so 1.25M lbf

2) SRB is 2.65 M lbf. Two boosters give 5.3M lbf.

3) That sums up to 6.6M lbf

4) Shuttle weighs 4.4 million lbs.

5) That works out to a 1.5 thrust-to-weight ratio at sea level launch.

If you still don't believe, watch a Shuttle launch and time how long it takes to clear the tower. You'll get the same answer (vertical acceleration at about 1.5g)