I took my DT-3 out for a ride a couple of weeks ago.  I have made a bracket to use a high fender and can usually see a "glow" on the fender, showing me the headlight is on and working.  When I was back from the ride, I didn't think I was seeing the glow on the fender, and sure enough, no headlight.
I took the rim and bulb out and was actually able to determine that both filaments were still working (whew!-$) using 2 AA batteries and a couple pieces of scrap wire.  With the bike running, the taillight WAS working, so I knew the bike was generating current.  I was contemplating taking the main key switch apart to check for loose solder joints, I've seen and repaired this before on other bikes.  I then noticed the two fairly heavy gray sheaths in the headlight bucket and followed them back to the left-hand hi/low beam, turn signal and horn switch enclosure.  I noticed the hi-low beam slider was sitting kind of wonky and tried moving it several times, then took it apart.  There are a couple of small brass contacts in there.  I think the switch moved sideways just enough that the contacts weren't making contact with the ends of the wires.  Anyway, I didn't really do anything specific, other than move the brass contacts a little with my finger, but I put it back together and have a working headlight again.
I'm thinking if I leave the switch slider alone, I probably won't have trouble.  If it goes out again, I know where to start.
If it gives me more trouble consistently, I will probably just solder the two wires for low beam together.  I don't ride at night, but it is law where I live to have a working headlight on the street.
Hoping this might help someone if they have a similar problem.

Thanks Phyllo!  Great stuff!

Just a side note on the "law", in most (not all!) states the headlight being on during the day is actually dependent on the year the motorcycle was manufactured.  Same thing with having turn signals installed (turn signals are generally not required for bikes manufactured before 1973).

Best to always ride with it on....  I just wanted to mention this because if you ever forget and get stopped, it could get you out of a ticket if you have an earlier bike.  They could possibly give you a ticket if your headlight doesn't work at all, but it's not a ticket in many places if headlight works and you just didn't have it on on an older bike.  (generally pre-78 or so as that's about when the Feds required an "always on" headlight)  No need to argue with the officer.  But research the law they cite you under and the court might throw out the ticket.

Thanks Mark T - also great stuff! I would actually want to run my headlight for the additional safety, but good to know!

Quick follow up - the next time I went for a ride (yesterday), bike fired right up, no headlight. I wiggled the hi/low slider a bit, headlight came back on and stayed on for the duration of my ride. Looks like it’s time to heat up the soldering iron…

Could be that the parts are dirty and/or tired.

Give it a strip and clean, but take care when removing the hi/lo toggle (need to remove the screw that goes through it), as the inner toggle can get quite brittle. I can't be certain, but I think that there is a small ball bearing and spring in there. Some of the same era Kawasakis used basically the same switchgear and, as they offered the individual components as individual spares, maybe checkout the CMSNL and Partzilla sites.

Edit

Check out this old thread and this link to CMSNL.

CT3 left handlewire switch button - Page 2 - Vintage Enduro Discussions (yamaha-enduros.com)

Kawasaki F7 1971 USA HANDLEBAR (F7/F7-A) - buy original HANDLEBAR (F7/F7-A) spares online (cmsnl.com)