I measured one dependent variable between three groups of data (control, group1, group2).The result of ttest indicates significant difference between control and group 1 (p vale =.04), control and group 2(p value .01), but not significant between group 2 and group 3.Do I still need to use the post hoc test? If so, which test is the most appropriate?

If you've got more than two groups, don't use ttests (due to the multiple comparisons trap). Use a oneway ANOVA instead (based on an Ftest). Then if you want a posthoc test after that, use Tukey's HSD. 

