How did French colonial rule affect Vietnam Cambodia and Laos in the years before independence

Essentially, the effect was negative.

French Indochina, the name of the Colony that encompassed the three modern countries, was called a Colony of Exploitation, which means that the French had no interest in investing in it, but only exploited its resources. They established a monopoly on any trade in the region, and forced the locals to pay taxes to them, without investing these taxes into the local infrastructure.