1) I think there is some hour error/mistake on my time sheet.
2) There is an error on the total hour on my tomesheet.
3) Did you work Saturday of 3 weeks ago? (the saturday which belong to 3weeks ago).
1) I think there is some mistake about the hours on my timesheet.
2) There is an error on the total hours on my timesheet. (Use mistake or error; there's not much difference.)
3) Did you work on Saturday three weeks ago - 29th March? (Stipulate the date to avoid any confusion.)