As an employer, you can use either standardise the shift allowance or pay your employees according to their rostered shifts.
To standardise the shift allowance rate, work out the average:
Your company pays no allowance for the first shift, $5 for the second shift, and $10 for the third shift. Therefore the average allowance per shift is: (0+5+10) / 3 = $5
If an employee takes leave, they should be paid an allowance of $5 for every day they're on leave, regardless of what shift they were rostered for.
To pay your employees according to their rostered shifts, pay them as though they were at work.
Your company pays no allowance for the first shift, $5 for the second shift, and $10 for the third shift.
If an employee goes on leave while being rostered for the first shift, they receive no allowance. However, if an employee goes on leave while being rostered for the third shift, they receive $10 per day.
You can use other methods as well, as long as they are fair and reasonable.