  Atmospheric compositions can provide powerful diagnostics of formation and
migration histories of planetary systems. We investigate constraints on
atmospheric abundances of H$_2$O, Na, and K, in a sample of transiting
exoplanets using latest transmission spectra and new H$_2$ broadened opacities
of Na and K. Our sample of 19 exoplanets spans from cool mini-Neptunes to hot
Jupiters, with equilibrium temperatures between $\sim$300 and 2700 K. Using
homogeneous Bayesian retrievals we report atmospheric abundances of Na, K, and
H$_2$O, and their detection significances, confirming 6 planets with strong Na
detections, 6 with K, and 14 with H$_2$O. We find a mass-metallicity trend of
increasing H$_2$O abundances with decreasing mass, spanning generally
substellar values for gas giants and stellar/superstellar for Neptunes and
mini-Neptunes. However, the overall trend in H$_2$O abundances, from
mini-Neptunes to hot Jupiters, is significantly lower than the mass-metallicity
relation for carbon in the solar system giant planets and similar predictions
for exoplanets. On the other hand, the Na and K abundances for the gas giants
are stellar or superstellar, consistent with each other, and generally
consistent with the solar system metallicity trend. The H$_2$O abundances in
hot gas giants are likely due to low oxygen abundances relative to other
elements rather than low overall metallicities, and provide new constraints on
their formation mechanisms. The differing trends in the abundances of species
argue against the use of chemical equilibrium models with metallicity as one
free parameter in atmospheric retrievals, as different elements can be
differently enhanced.
