is it possible to assign multiple tasks (i.e.: Formwork, Reinforcement, Pouring Concrete) with different Appearances Profiles to only one 3D resource?
Yes, a single resource can be assigned to as many tasks as you wish. The Appearance Profile used can be different for each resource on each task (ie. a single resource can use different profiles on different tasks, and multiple resources can use different Profiles on a single task)
You can change the Appearance Profile in the Quick Access toolbar before assigning Resources, or you can change the Profile after assignment in Task Properties>Resources