There are 3 spacers with a HTII, each 2.5mm. Two are needed (one each side) if you have a 68mm shell width, to space it out to 73mm which is the other BB shell size (for threaded type BB’s, let’s ignore oddities and push fits for now). 68 + 2*2.5 = 73.
The cranks are sized for 73mm so as above if you leave the spacers out then they’ll have 5mm of side to side play and won’t load the bearings.
The third spacer is needed with both 68 and 73mm, and is there to use up the space that an e-type mech fitting would take. If you have an E-type mech then you don’t need this third spacer.
So in your case, with no mech and a 68mm shell you need all three.
Where you put the spacers is up to you – you can tune chainline a bit with them, but only in 2.5mm steps, and if you move from the 2DS / 1NDS then your cranks will not be equidistant either side of the BB shell. Whether you’d notice 2.5mm or even 5mm on an mtb is a moot point though. Just be aware if you move the cranks inbound on the DS by moving spacers to the NDS that you don’t run into clearances issues with granny tabs, etc.