Are level 3 and level 6 papers just pass/fail?

[richmal]

Is the level 3 paper taken in year 2 just a pass/fail and therefore allocated a 3c, or do higher marks equate to 3b or 3a?

Similarly, do higher marks on the level 6 paper taken in year 6 only allow a 6c to be indicated or are there band levels for 6b and a?

No

There are level 3-5 paper and depending on the marks acquired and the threshold for each level the level is calculated. The level 6 is a separate test.
Technically sub levels don't exist