Excel Problem: Counting if any 3 Dates in a column are within 300 days of each other

[trobb001]

I am currently working on a spreadsheet to calculate values depending on whether or not they meet specific criteria.

I need a formula to workout whether or not there are any 3 FTE's within 300 days of other, as this is the criteria that I require.

This box is a simplified version of the main criteria, below I have put the entire formula that I use.

Type	Source	School	Field 1	Field 2	Field 3
FTE	A	ABC High	12/09/2013	8	-
FTE	A	ABC High	30/05/2014	2	-
PERM	A	ABC High	06/06/2014	-	-
SEN	B	BCD High	09/06/2014		-
CME	B	Not at school	10/08/2014		-
FTE	A	BCD High	04/08/2014	8	-

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Currently, I use this formula to workout whether or not a value should be counted. But what I need is a way of working out if any 3 FTE values are within 300 days of each other (threshold), so I can then count the all FTE as events.
=(SUM(IFERROR(1/COUNTIFS($I$112:$I$161,$I$112:$I$161,$E$112:$E$161,">="&"41518",$E$112:$E$161,"<="&"41639",$I$112:$I$161,"<>P20000000",$D$112:$D$161,"<>0",$C$112:$C$161,"<>0",$B$112:$B$161,"=FTE",$E$112:$E$161,"<>0", $F$112:$F$161, "<>0"),0)))

I think that maybe I might have to have a separate column to have an indicator that I can then just put an extra COUNTIF criteria in to look for a value.

Any help on this issue would be greatly appreciated!