Remember that the Home Hub 3 will set the speed of the wireless to the lowest speed device you have connected. So, if you have a naff old laptop with a wirelss g interface, then your super duper laptop with wireless n will still get wireless g speeds - ie a tad under 30 megs max. If you are all wireless n, then you may get the full speed your line can deliver ... but the caveat is that there are a number of variables which mean your speeds can be much lower than you expect.
Not sure about the homehub but this is a wide misconception nowadays. The original N routers and quite a few draft N routers would slowdown to the lowest common denominator.
Nowadays most N routers can handle simultaneous G and N connections without slowing the N band down, provided WPA2-PSK AES encryption is used by all.
That said if a wireless B device is introduced onto the network then everything slows down terribly as virtually no router can handle B and N.
G and N generally ok. B and N not ok.
With the default Sky router I have a mix of all devices.
ON G: sky wireless connector, 2 laptops, 1 wifi printer and a games console, possibly the blackberrys
On N: Few macbooks, iphones and a new pc.
The N devices still get around 75Mbps throughput when I test next to the Sky router. As I say things have definitely changed in this respect. ALTHOUGH some routers STILL cannot support N and G as you say.