Between 3-chlorophenol and 3-fluorophenol, which one is more acidic?
Experimentally, the $\mathrm pK_\mathrm a$ values for 3-chlorophenol and 3-fluorophenol appear to be (perhaps unexpectedly) equivalent.

The table below1 shows the measured $\mathrm p K_\mathrm a$ values of substituted phenols in dimethyl sulfoxide (DMSO):

$$\begin{array}{ccc} \hline
\text{substituent} &amp; \sigma &amp; \mathrm p K_\mathrm a \\ \hline
\ce{H} &amp; 0 &amp; 18.03 \pm 0.04, \color{silver}{16.4} \\
\ce{p-CH3} &amp; -0.170 &amp; 18.86 \pm 0.05 \\
\ce{m-CH3} &amp; -0.069 &amp; 18.23 \pm 0.01 \\
\ce{p-Cl} &amp; 0.227 &amp; 16.74 \pm 0.03 \\
\color{red}{\ce{m-F}} &amp; \color{red}{0.337} &amp; \color{red}{15.88 \pm 0.07} \\
\color{red}{\ce{m-Cl}} &amp; \color{red}{0.373} &amp; \color{red}{15.83 \pm 0.05} \\
\ce{m-CN} &amp; 0.56 &amp; 14.76 \pm 0.05 \\
\ce{m-NO2} &amp; 0.710 &amp; 14.39 \pm 0.12, \color{silver}{13.75} \\
\hline
\end{array} \\
\small\text{(Greyed out values are taken from ref 2.)}$$

In general, halides have slightly unusual effects on aromatic rings. Despite being highly electronegative (and deactivating), they actually donate into the pi system via their lone pair. 

This donation means that the phenoxide generated upon deprotonation (i.e. the dissociation being measured by the $\mathrm pK_\mathrm a$ value) isn't as stabilised as it would be with a purely withdrawing group (such as the nitrile or the nitro groups). 

References


Bordwell, F. G.; McCallum, R. J.; Olmstead, W. N. Acidities and hydrogen bonding of phenols in dimethyl sulfoxide. J. Org. Chem. 1984, 49 (8), 1424–1427. DOI: 10.1021/jo00182a020.
Chantooni, M. K., Jr.; Kolthoff, I. M. Comparison of substituent effects on dissociation and conjugation of phenols with those of carboxylic acids in acetonitrile, N,N-dimethylformamide, and dimethyl sulfoxide. J. Phys. Chem. 1976, 80 (12), 1306–1310. DOI: 10.1021/j100553a009.