  We present a new method of constructing a fully robust qubit in a three-level
system. By the application of continuous driving fields, robustness to both
external and controller noise is achieved. Specifically, magnetic noise and
power fluctuations do not operate within the robust qubit subspace. Whereas all
the continuous driving based constructions of such a fully robust qubit
considered so far have required at least four levels, we show that in fact only
three levels are necessary. This paves the way for simple constructions of a
fully robust qubit in many atomic and solid state systems that are controlled
by either microwave or optical fields. We focus on the NV-center in diamond and
analyze the implementation of the scheme, by utilizing the electronic spin
sub-levels of its ground state. In current state-of-the-art experimental setups
the scheme leads to improvement of more than two orders of magnitude in
coherence time, pushing it towards the lifetime limit. We show how the fully
robust qubit can be used to implement quantum sensing, and in particular, the
sensing of high frequency signals.
