Staining for CD3 in combination with CD4 and CD8 should be reliable enough. Depending on your staining panel, you can add Abs to exclude other cells (like CD19 for B cells) or add some more markers for further characterization of T cells (CD44 or CD45RO vs CD45RA as memory markers).
For a 'deeper' characterization I would consider staining for CD62l, CD11a, in combination CD43 and CD27, and again in combination CD127 and KLRG1 in addition to the suggestions above. It all depends on what you are after . . . if you just want to enumerate the percentage T cells in your isolate CD3, CD4 and CD8 will get you pretty close, add an NK marker to exclude NK T cells . . . and you're pretty much there. if you want to know more the above is a reasonable starting point I think.