In the interest of giving you some kind of a response, I'll put in my two cents' worth. I'm no Dumeril's Boa expert specifically, but I've been keeping reptiles almost my whole life - so I think this isn't too much of a stretch. This is a really slow forum at times.
First, it makes a difference in what kind of advice anyone can give you, if you can tell us how long you've had the snake, and more specifically, how long you've been feeding it by the regimen you describe. Also, since everybody under the sun has a different definition of prey sizes, some kind of a measurement (weight would be excellent) on the "small" rats in question would also be helpful.
That being said, I'll offer a note of caution: from what I have read here, and seen in my own (1) Dumeril's Boa, this species seems to have a somewhat shorter stomach and perhaps slower metabolism than a lot of other commonly kept species. DEPENDING on the size of the prey items, this may be perfect, or may be asking for trouble.
If you haven't seen any problems with your snake eating on this schedule you describe, there may not be any issues to worry about. That isn't a guarantee. I may be misunderstanding how long you've been feeding him this way.
In the interests of your expenses though, if you are reaching the end of a bag of frozen rats that are small enough he is readily eating 3 at each feeding, it would probably be smarter to move up a size or two, just so you are not wasting your money.
All of this is pure speculation. Things for you to consider. Hopefully you will get some more responses. 
~Rebecca
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0.1 Dumeril's Boa '04 (Courtney)
1.2 Ball Pythons
[1.0 '05 Orange Hypo (Specter)]
[0.1 '05 Het Orange Hypo (Sylvia)]
[0.1 '03 Normal (Sue)]
0.2 American Pit Bull Terriers (40lb darling lap dogs:Brandy&Mara)