Vietnam, Laos and Cambodia in May

Everything that is not on your list is "missing".

But...you only have 3.5 weeks.And you already have too many places for that ;-)

I would suggest Halong Bay

Ok, so i should skip Nihn Binh and rather go for Halong Bay?

Everything that is not on your list is "missing".

But...you only have 3.5 weeks.And you already have too many places for that ;-)

You might be right, what do you recommend to take out?

Depends on how many days you plan on each location. Your schedule is very full already and you will spend so much time transferring from one place to the other . I count 14 stops in 25 days. You should count all the time spent on busses, airports, flights, taxi too. You won't have much time to enjoy each location.
I would probably skip Phnom Pehn and Ninh Bihn (though I've never been here) at least. I wouldn't spend time in Danang, (just shortly for airport transiting). I don't know Laos, so i can't comment on that.

Txs
Danang is just airport:
Traansp plan is:
HCMC - plain to
• DaNang - buss to
• Hoi An - priv car to
• Hue - night train to
• Nihn Bihn / Halongby - buss to
• Ha Noi - plain to
• Luang Prabang - plain to
• Battambang - boat to
• Siem Reap - plain to
• Phnom Penh - plain to
• Phu Quoc Island - boat/buss to
• Can Tho
• Vihn Long - buss
• HCMC

What to leave out would depend on my interests,which may be different from yours.

Personally I'd want around 3 weeks for an introductory north-south Vietnam trip.

A week or more for that basic Cambodia segment.

And a couple of weeks for a quick Laos trip,but if only LP....about 4-5 days there.

Plus travel days between the countries....

This is way too much for 3.5 weeks, you'll have to travel every other day which is a huge nuisance, not to mention packing up, leaving your hotel, finding your new hotel. You'll literally spend half of your vacation time moving from one place to the other. That's one way to ruin a trip.