Your iPad 3 doesn't support 40MHz wide channels on the 5GHz band, which has a shorter range, so it takes the 2.4GHz signal, which is stronger and as far as the iPad is concerned, as fast.
The iPhone 5 does support 40MHz wide channels on the 5GHz band, but it's reception is balls, and hence it connects to the stronger 2.4GHz band as well.
Your iPad Mini has a decent antenna, hence is able to establish a reliable signal on the faster 5GHz band, but the signal is weaker than the one on the 2.4GHz band, which results in the two bars.
Getting a
Netgear Dual-Band WiFi extender will fix all of that - and you can take it with you, as well.