  Understanding nonthermal particle generation, transport, and escape in solar
flares requires detailed quantification of the particle evolution in the
realistic 3D domain where the flare takes place. Rather surprisingly, apart of
standard flare scenario and integral characteristics of the nonthermal
electrons, not much is known about actual evolution of nonthermal electrons in
the 3D spatial domain. This paper attempts to begin to remedy this situation by
creating sets of evolving 3D models, the synthesized emission from which
matches the evolving observed emission. Here we investigate two contrasting
flares: a dense, "coronal-thick-target" flare SOL2002-04-12T17:42, that
contained a single flare loop observed in both microwave and X-ray, and a more
complex flare, SOL2015-06-22T17:50, that contained at least four distinct
flaring loops needed to consistently reproduce the microwave and X-ray
emission. Our analysis reveals differing evolution pattern of the nonthermal
electrons in the dense and tenuous loops; however, both of which imply the
central role of resonant wave-particle interaction with turbulence. These
results offer new constraints for theory and models of the particle
acceleration and transport in solar flares.
