You can check your signal, as well as whether there are any maintenance issues in your area, in the help section of our app.
You can also check your signal quality using Ofcom's signal checker here: https://checker.ofcom.org.uk/en-gb/mobile-coverage
Or with Three's signal checker here: http://www.three.co.uk/Discover/Network/Coverage
To check whether there's maintenance work going on on masts in your area enter your postcode here: http://www.three.co.uk/support/network_and_coverage/network_support