  We use 3D MHD simulations to investigate nonlinear development of the Parker,
magneto-Jeans (MJI), and swing mechanisms in galactic disks. The model disks
are local, isothermal, and begin from a vertically-stratified equilibrium. We
first construct axisymmetric equilibria and examine their stability. Finite
disk thickness reduces the critical Toomre Q parameter below unity; we find Q_c
\~ 0.75, 0.72, and 0.57 for \beta=\infty, 10, and 1 cases, respectively. We
then pursue fully 3D models. In non-self-gravitating cases, the peak density
enhancement from the `pure' Parker instability is less a factor of two. The
dominant growing modes have radial wavelengths comparable to the disk scale
height H, much shorter than the azimuthal wavelength (~10-20 H). Shearing
disks, being more favorable to midplane-symmetric modes, have somewhat
different late-time magnetic field profiles from nonshearing disks, but
otherwise saturated states are similar. Late-time velocity fluctuations at 10%
of the sound speed persist, but no characteristic structural signatures of
Parker modes remain in the new equilibria. In self-gravitating cases, the
development of density structure is qualitatively similar to our previous
results from thin-disk simulations. The Parker instability, although it may
help seed structure or tip the balance under marginal conditions, appears to
play a secondary role. In shearing disks with Q less than a threshold level ~
1, swing amplification can produce bound clouds of a few times the local Jeans
mass. The most powerful cloud-condensing mechanism appears to be the MJI. Our
simulations show that condensations of a local Jeans mass (~3\times 10^7 M_sun)
grow very rapidly, supporting the idea that MJI is at least partly responsible
for the formation of bound cloud complexes in spiral galaxies.
