First, as with any question concerning prophecy, the answer is always "Maybe." It largely hinges on whether you think the prophecy has already occurred or not, and also on whether you think the prophecy will occur relatively quickly or not. Since you seem to be asking from the perspective that it has not occurred yet and will occur relatively quickly I will attempt to answer from that perspective.1 2
The G-7, or Group of Seven, is primarily concerned with world economics, and has historically been comprised of nations that contain at least 50% of the world's wealth. The idea that a financial group could be the beast that Revelation refers to, in light of the kind of power that the beast is later said to have, does not seem far fetched. Consider this quote generally attributed to a very prominent international banker of the 19th century.
"Give me control of a nation's money and I care not who makes it's laws" — Mayer Amschel Bauer Rothschild
However, this does not pinpoint the G-7. There are many financial groups that hold very much power over certain nations and their population. With some purposeful ambiguity '7' and '3' of anything can be found in any group. The Federal Reserve, for example has seven members of its Board of Governors. One might suppose that the incorporation of three more is the beast. I would not be surprised if this theory does legitimately exist, but here and now I independently conceived it. Which shows my point.
The inclusion of these three ambiguous nations into the G-7 is just as ambiguous as the inclusion of three hypothetical persons into the Board of Governors. Especially considering there has already been a G-8 as well.
All this is said with a healthy amount of skepticism, but not total convincing that there is no way the G-7, The Fed, or any other financial institution is not the beast. As mentioned at the top, it is really hard to tell what prophecy is referring to until it has already occurred.
- For personal edification I urge you to consider this prophecy as one that might have already happened or is even happening now. I think this idea is largely ignored by Christians.
- Misc Notes about Revelation: This particular book is filled with prophecy. Literally, it is almost entirely prophecy which is what makes it so hard to understand. There is a lot to consider when evaluating something as fulfilling a part of Revelation, therefore, I say that your question is a bit loaded and surely at least 20 pages could be written on the possibility of G-7 being the beast.