Two diagonals that differ by 8 mm on a 3 metre frame means the corner is off by about a degree — enough to see. Enter your sides, get the diagonal it should measure, and if you have measured both, exactly how far out you are and which way.
Notes
Why diagonals prove square
A rectangle is the only quadrilateral with equal sides and equal diagonals. Sides alone do not prove it: a parallelogram can have four correct sides and still lean. Equal diagonals are the proof.
How to correct it
Push the corner at the end of the longer diagonal towards the shorter one. Half the difference is roughly how far the corner moves — so an 8 mm difference means nudging about 4 mm.
The 3-4-5 method
Any triangle with sides in the ratio 3:4:5 has a right angle. It works at any scale, and bigger is more accurate — measuring 300:400:500 is far more forgiving than 30:40:50.
What this does not do
It checks a rectangle in one plane. Winding — a frame twisted out of flat — is a separate problem and needs winding sticks, not a tape.