Grainne,
If what you mean is that you pay mortgage payments 3 months later than you should have then the cost is interest on the 3 months payments ie not a lot.
eg if your payment was 1,500 per mth = 4,500 works out about circa 50 cost extra.
If the payments eg 3 @ 1500 are added as an extra 3 months to mortgage it means that 4500 gets interest each month of circa e15 until mortgage is paid off..