Can't connect to 3G in certain places?

I have an unlimited internet contract on my iPhone 4S, I can browse freely most of the time, but when I go to university, the phone changes from 3G to 3. I think it is trying to connect to the university internet, which I don't want to use or it can't connect to 3G.

How can I stay on 3G when I'm there?

Added (1). I'm in the UK, I'm assuming the area is covered because I can connect outside the uni, just can't when I'm inside…

Many areas may not have 3G towers in range
you do know, if you connect to the scool's Wi-Fi, it will be SIGNIFICANTLY faster.

Are you within the US? If so, that's pretty strange. In fact all US are covered by 4G signal. Your iphone could have any problem

You just have a weak signal inside the building that's all, if you can connect to Wi-Fi while inside the building.
That's what I do while at work because inside the building we have a very weak signal on 3g so for me Wi-Fi is the answer if I need to connect.

If you want to only use the 3G signal (if available) then disable Wi-Fi but leave Data enabled.

However, in buildings
and especially university buildings expect that the 3G signal will be very weak
Old buildings usually have thicker walls
and new buildings metal re-inforced concrete and thick at that.
which blocks signals.
3G signals find it the hardest to get into buildings, which is why '3' has to do offers like unlimited data.

if you are seeing only the '3' symbol, then it probably has only enough signal to identify the network - and possibly do calls, but not enough to do data (or much data).

In all honesty, if you have Wi-Fi available I would use it - it is likely to be faster than most 3G signals (unlimited allowance or not).

If it was connected (not trying to but actually connected) then you would see it show the Wi-Fi symbol and the name of the network it was connected to.