The maximum threshold of IBUs in a given batch of beer is 100, anything over that is only adding to flavor (and head retention and mouthfeel). This maximum number is also only in worts of around 1.050. Now if you are boiling 3 gal down to about 2.5, and adding 2.5 gal of wort, then your maximum is already sliced down to 50 IBUs in a batch, and that is still with a wort gravity of 1.050 or below.
Now, with a full boil if you are trying to reach a final gravity 1.050, then you will have near 100% efficiency when it come to alpha acid isomerization and IBU absorption, and you can reach the 100 IBU mark, since you would be starting the boil around 1.044. When using LME, this will be about 7 lbs. If you put 7 lbs of LME into only 3 gallons of wort, then you are starting with a pre boil gravity of 1.084, boiling down to probably around 2.5gal at 1.100 before topping of with water to reach your OG of 1.050. Far above the ideal IBU efficiency gravity. This will leave you with a wort that has a max IBU rating of approx. 82.7, not too bad, but wait. After topping off, you will have only 41.4 IBUs.
So without doing a late extract addition you maximum IBU level would be 41.4, not quite the hop bomb you may have thought you were making.
Now, if you add say, half at the end, lets see what that does. Your pre boil gravity will be around 1.042 (very close to that of a full wort boil), and you will boil off around to about 2.5 gal at 1.050, perfect. So now you have 100 IBUs, top off and you are at you maximum of 50 IBUs, still not the hop bomb you thought, but better. If you are only adding 2 gal of top off water, then you max IBU will be 60, 1.5 gal and the max IBU is 70, 1 gal is 80, .5 gal is 90 and not topping off can get you the maximum IBU level of 100.