For last 3 characters:
ORDER BY RIGHT(id,3)
For a *number* after the slash (and allowing the number to be any length but wanting them sorted *as* numbers), it's more complex but doable:
ORDER BY CONVERT( REVERSE( LEFT( REVERSE(id), INSTR( REVERSE(id), '/' )-1 ) ), DECIMAL )
I'll explain that one.
REVERSE(id) will take "09/09/PG-5/117" and convert it to "711/5-GP/90/90"
INSTR( "711/5-GP/90/90", '/' ) will return 4, the position of the first / in that string
LEFT( "711/5-GP/90/90", 4-1 ) thus gets us "711"
Which we REVESE again to get 117.
And we can then convert 117 to a number (decimal).
And now "09/09/XY-1/1137" *will* come out as larger than "09/09/AB-4/73" which it wouldn't if we just did ORDER BY the *strings* "1137" and "73".
You pays your money and takes your choice.