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There is a 28 days grace period which means that the Home Office will disregard the 3 days you overstayed. You should not give excuses but you may be required to say why you overstayed.
If you are asked to give the reason for overstaying you must say why. I am not suggesting you would but you must not mislead or attempt to mislead if you were asked to give reasons.
I am happy to answer further questions when previous questions have been rated.
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If you made your application before your visa expires then there would be no requirement for a 28 days grace period. I really cannot go on and on about this anymore as I have explained in numerous answers the 28 days grace period is in place for those who have made an application after their visa has expired but before the end of 28 days not to be penalised as would be the case for overstayers.