The Contrast table is just looking at doing two sample tests for (mean_a,mean_b), (mean_a,mean_c), and (mean_a,mean_control) where (x,y) looks at x - y and tests whether there will be some kind of statistical significant effect of the two means not being equal (or not).
The contrast framework just looks at linear combinations of different treatment means and for this special case it is a difference of the other means with respect to the mean of group a.
It looks like you will need to do some kind of ANOVA/T-test/Other test on the individual means if the original H0 ANOVA was rejected (i.e. all treatment means the same) to test whether you get significant differences in pair-wise comparisons.
Also can you point out what a Dunn test is?