  We present a theoretical analysis of rf association of Efimov trimers in a
2-component Bose gas with short-range interactions. Using the adiabatic
hyperspherical Green's function formalism to solve the quantum 3-body problem,
we obtain universal expressions for 3-body rf association rates as a function
of the s-wave scattering length $a$. We find that the association rates scale
as $a^{-2}$ in the limit of large $a$, and diverge as $a^3 a_{ad}^{3}$ whenever
an Efimov state crosses the atom-dimer threshold (where $a_{ad}$ stands for the
atom-dimer scattering length). Our calculations show that trimer formation
rates as large as $\sim10^{-21}$ cm$^6$/s can be achieved with rf Rabi
frequencies of order 1 MHz, suggesting that direct rf association is a powerful
tool of making and probing few-body quantum states in ultracold atomic gases.
