  The three-qubit Toffoli gate plays an important role in quantum error
correction and complex quantum algorithms such as Shor's factoring algorithm,
motivating the search for efficient implementations of this gate. Here we
introduce a Toffoli gate suitable for exchange-coupled electron spin qubits in
silicon quantum dot arrays. Our protocol is a natural extension of a previously
demonstrated resonantly driven CNOT gate for silicon spin qubits. It is based
on a single exchange pulse combined with a resonant microwave drive, with an
operation time on the order of 100 ns and fidelity exceeding 99%. We analyze
the impact of calibration errors and 1/f noise on the gate fidelity and compare
the gate performance to Toffoli gates synthesized from two-qubit gates. Our
approach is readily generalized to other controlled three-qubit gates such as
the Deutsch and Fredkin gates.
