  We combine multiple campaigns of K2 photometry with precision radial velocity
measurements from Keck-HIRES to measure the masses of three sub-Neptune-size
planets. We confirm the planetary nature of the massive sub-Neptune K2-182 b
($P_\mathrm{b}= 4.7$ days, $R_\mathrm{b} = 2.69$ $R_\oplus$) and derive refined
parameters for K2-199 b and c ($P_\mathrm{b} = 3.2$ days, $R_\mathrm{b} = 1.73$
$R_\oplus$, and $P_\mathrm{c} = 7.4$ days, $R_\mathrm{c} = 2.85$ $R_\oplus$).
These planets provide valuable data points in the mass-radius plane, especially
as TESS continues to reveal an increasingly diverse sample of sub-Neptunes.
K2-182 (EPIC 211359660) is a moderately bright ($V = 12.0$ mag) early-K dwarf
observed during K2 campaigns 5 and 18. We find K2-182 b is potentially one of
the densest sub-Neptunes known to date ($20 \pm 5$ $M_\oplus$ and $5.6 \pm 1.4$
g cm$^{-3}$). K2-199 (EPIC 212779596; $V = 12.3$ mag) is a K5V dwarf observed
in K2 campaigns 6 and 17 which hosts two recently-confirmed planets. We refine
the orbital and planetary parameters for K2-199 b and c by modeling both
campaigns of K2 photometry and adding 12 Keck-HIRES measurements to the
existing radial velocity data set ($N$ = 33). We find K2-199 b is likely rocky,
at $6.9 \pm 1.8$ $M_\oplus$ and $7.2^{+2.1}_{-2.0}$ g cm$^{-3}$. K2-199 c has
an intermediate density at $12.4 \pm 2.3$ $M_\oplus$ and $2.9^{+0.7}_{-0.6}$ g
cm$^{-3}$. We contextualize these planets on the mass-radius plane, discuss a
small but intriguing population of "super-dense" sub-Neptunes ($R_\mathrm{p} <
3$ $R_\oplus$, $M_\mathrm{p} > 20$ $M_\oplus$), and consider our prospects for
the planets' atmospheric characterization.
