To answer your question using basic figures, the 10 PCs will share the outgoing/incoming internet connection (3Mbps). Each PC has access to the full 3Mbps and if only 1 PC is downloading at any point in time then it will download at 3Mbps. If 2 PCs are downloading at the same time they will download at 1.5Mbps each, etc. etc. This will only really become apparent to the user if each is actually downloading a file or streaming data at exactly the same time, in a normal web browsing or email environment (i.e. not real time) the fact that the connection is shared will hardly be noticed, it may slow down a bit at lunchtimes though.
Where I work we have a 2Mbps connection between about 15 PCs and it works fine.
I agree with AC though, wireless networking is not really suitable for a business environment, especially if large amounts of data is being trensferred between PCs.