Hi,
one month for the three countries and also getting to Bangkok will leave you with little time between getting from one place to another. I really think you should focus on either one country or at least one region, Northern Vietnam and Northern Laos, for example, or Southern Vietnam and Cambodia.
In September you might experience a lot of rain, depending where you are. Better check the weather details for each country, because with a lot of rain hiking won't be a lot of fun, even if you are thinking more of shorter half-day or day hikes.
Leaving the weather apart, I think of Northern Vietnam and Northern Laos when I hear "hiking". Terrific mountaineous regions. Southern Laos and Cambodia are rather flat. Cambodia might offer some walking to some ruins, although I haven't heard that there is a lot on offer.
Besides the points mentioned above, Laos and Cambodia (and to a minor extent Vietnam) continue to suffer from a not so pleasant heritage. The Vietnam war converted Laos into the most heavily bombed country in the history of mankind, Cambodia was flooded with mines during the war and the Pol Pot aftermath. Unexploded ordnances (UXO) are still in the ground in many regions in Laos. At the present clearing speed it will take almost hundred more years to get the country 'clean'. So it's almost logical, that may parts of Laos are off limits for outdoor activities. Same with Cambodia, where many areas which are not explicitely cleared of mines, have to be considered as too dangerous to move around freely. So not that many opportunities for hiking in these two countries.
Suggestions where to go... here are some of my favorites:
… Angkor Wat, of course, and not less than three days
… the border area of Northern Cambodia, before crossing into Laos.
… Bolaven Plateau in Southern Laos (quiet and a bit secluded)
… Phonsavan in Northern Laos. Most people go there for one day only to see the Plain of Jars. I got to know some local youths, was invited to their english lessons and came even back months later for some more days. Quiet place and possibilities to do a bit of walking
… Savannakhet. Fell in love with this town, nobody understands it
… Mekong Delta. I'm very independant-minded and hate everything that smells of organized tour, but I thoroughly enjoyed the three-day tour of the Mekong Delta.
… Hoi An, Vietnam
… and above all: Champasak, a tiny place in Southern Laos, bit south of Pakse, on the other side of the Mekong. There you can get a bicycle and cycle around 2 or 3 days and see some very simple yet stunning images of Southeast Asia. If you think you are interested, I can pm you a description of which direction to take to get around...
And again: don't try to see three countries in one month...
Have a great trip
Micha