Assurance that their jobs are safe is a joke. Its like when a company says that they are just one big family. Only management believe those lies and they force them on everyone else.
Are you truly being honest about keeping them on? Write up a contract that guarantees them X years of employment, at Y severance per month if terminated early.
More realistically, you should consider that the ones resigning have a better read on the situation than you do.
The reaction of those 3 employees was my own, and when the culture is so clearly against you, and the company clearly doesnt have more than fluff words (across the org) to retain you, then the net sentiment has shifted strongly away from "trust the employer" to "the employer will screw me the first chance they get".