I need a little advice on splitting crank cases. I've never split the cases on an old Yamaha.  Is there a way to not pull the trans. all apart.  What I'm doing is replacing crank bearing and seal.  Thanks for any advise.

In the long run its probably better just to dismantle it all. You say crank bearing & seal so i'll take a guess you mean the left side. So re-thinking your question, you 'could' do just that by--mag off--sprocket off--selector drum retainer off under the three screw cover above the sprocket--detent neutral plunger & spring on top at an angle. Neutral light switch--except i don't think they have one?--circlip & washer off the change shaft. What have i forgotten anyone??. Then all the screws out--don't miss any. Then need a special tool to pull the case likely with bearing away from the crank & from gear shafts & everything else. If you get that far you'll need to tap the sprocket shaft softly plus help the case that end so it comes off square on. If ya super lucky refitting it--with another special tool--you'll need to be very careful to keep the change shaft pulled towards you or it'll hop off the drum pins. If that's not put you off then by all means go for it. Think i'd go the full dismantle to be honest. Link below for a look at all the parts.
www.partzilla.com/catalog/yamaha/motorcycle/1973/ct3

Well thanks for not sugar coating it. I've did several 500 tt&xt bottom ends & other brands bikes, but this is my first little yamaha. I've always been able to leave trans. intact of one half of case. Thank you very much i'll get it done. Your right it is the left side bearing bad.
 

RT325 has explained it well...  

Use simple two bolt puller to press left side of crank...  Yamaha provided two 8mm holes by ignition for this.  If you leave clutch and kick idler gear on, should keep everything in place pretty well.  Just be mindful of shift drum under three bolt cover RT325 mentioned...  Don't tap on shift shaft too hard.  If you leave clutch pushrod in you can top on that Go slow and you should be fine.

Once half is off, the crank should slide out of right slide bearing if you removed gear first.  You may not "need" to remove crank...  but it will be easier this way if you do.  

Left crank bearing normally stays on crank and need a separator to remove...  usually easier to do with crank removed.

Be mindful that sometimes crank spins inside bearing and worn crank stub is the cause of excessive play (if that's your issue0  

Auto parts store often rents a puller for free...  steering wheel puller will work...  might need longer 8mm bolts.  

Good luck!

Yes i forgot bearing usually stays on the shaft, so another problem. I have a bearing tool that can be clamped up behind it if you can get in there to get it started, then tighten the bolts to pull it together in & behind the bearing. Also, in rare cases, the bearing could have spun on the crank so just one more thing to check or the new bearing will go on tight to the last bit then be loose. You'll soon know as you'll be able to turn the inner race before you start removing it.
If you're used to 4 strokes, Yamy 2 pokes don't run a center gasket, just sealant. Sounds like you wanting to do it without removing the clutch cover or anything on the right side. Lets know updates please.