Well if by class you mean just skill lines, there's plenty that are fine to take a few points in. If you mean like major archetype skill lines like magic, warrior, archer, rogue, then generally focusing on one is better but you can do hybrids and classes that have the same stats mix up decently well(so marksmanship+scoundrel and all magic schools).
For warriors, I'd work on maxing Man at arms first. Other stuff isn't particularily important early on, the weapon skills scale fairly poorly past the first 2points(it's still 10% per skill but since cost increases, it becomes less and less cost effective), shield isn't really a necessity as long as you have an actual shield equipped, Willpower/Bodybuilding are nice to have but are more lategame options. Maxing man at arms will open up the skills that will let you deal with more situations. You could add a splash of magic to them at some point though you need enough int to cast magic without failling(which iirc is 8 for first tier, then +1 int per tier and you also need to increase the magic school to not get an AP penalty on the spells). I'd say it weakens the warrior thing but it gives you more options.
For the ranger, same thing, max Marksmanship, the talent at 5pt marksmanship is insanely good(-1 AP for bow attacks) so you want this asap. I'd probably grab 1pt in Scoundrel for Fast Track and like maybe Trip. Make sure you find the Fast Track book first, it's not a common one iirc. A potentially good option too is to get Guerilla talent and put a few points in sneak(2) then get some sneak gear and sneak before you fire. Being an archer you should be able to get in sneak even without being behind enemies by moving to the side of the battlefield out of the cone of sight and that will increase damage a lot. Though it's not really necessary and requires quite a bit of work.
For the spellcaster, get 1 in everything, buy the good lvl 1 spell, I made a fairly detailed list several posts back(look for giant posts a few pages back). You can't really go wrong with casters, maxing any school is fine, just need to max stuff that fits your playstyle. Air is control/damage but can hurt your melees with blood getting electrified, Water is control/heal, Fire is mostly pure damage, Earth is pets/control with some conditional damage(lots of mobs resistant to poison) and Witchcraft is buff/debuff/pets.