No. There are only six different directions you can thrust, but your acceleration, which is a sum total of the forces produced by all of your thrusters, can point in any direction.
When you press your controls for 'thrust right' and 'thrust up' simultaneously, you are only thrusting up and right separately (those are the directions your physical thrusters point) but your acceleration vector will point towards up and right in a diagonal angle, as it's the sum of the thrust vectors produced by your bottom and left side thrusters.
There are no limitations to the amount of directions you can accelerate, if you bind all three translational axes to analogue controls.
Even if you had all of them on the keyboard, you could still accelerate in a total of 26 different 'ways' relative to your ship.