Events that happen in certain time window

I have a data of about 4,000 corporate acquisitions, name of the acquirer and their announcement dates. I need to narrow this sample down to acquisitions that have been made by certain acquiring company during a 3 year time window. What is the easiest way to do this? To make matters more difficult, the dates are not in date format, but in a custom format, for example 20.04.93 etc.

Thanks in advance!

I attached example with 2 companies. In this case I would need to be able to exclude the acquisitions made by Acal PLC, because the company hasn't made 5 or more acquisitions during a 3 year time window, but I would need to keep companies like 365 Corp PLC in the sample because they have.

Thats not quite it, as I have quite a big sample with over 450 acquiring companies. What I would like my end result to look like is a list with only those companies and acquisitions that fit the following criteria:
1. Acquisitions are made by the same company
2. There are at least 5 or more acquisitions...
3. During a 3 year time window

I.e in the sample I posted, I need to somehow remove companies like Acal PLC from the list, while keeping companies like 365 Corp PLC. Unfortunately I can't post the whole list because I don't own the rights publish it.

Here is an example of what I mean, hope it illustrates the description a little better.