From wikipedia:

The passive voice in Swedish is formed in one of four ways:

add an -s to the infinitive form of the verb

use a form of bli (become) + the perfect participle

use a form of vara (be) + the perfect participle

use a form of få (get) + the perfect participle

Of the first three forms, the first (s-passive) tends to focus on the action itself rather than the result of it. The second (bli-passive) stresses the change caused by the action. The third (vara-passive) puts the result of the action in the centre of interest:

Dörren målas. (Someone paints the door right now.)

Dörren blir målad. (The door is being painted, in a new colour or wasn't painted before.)

Dörren är målad. (The door is painted.)

behandlet is not a word in Swedish. The plural is behandlade, singular is behandlad (common gender) or behandlat (neuter).

Jag blev väl behandlad I was treated well
Vi blev väl behandlade We were treated well
Ämnet blev behandlat … The subject was treated …

behandlad is the singular en form of the word, but vi is plural.
Vi blev väl behandlade 'we'
Han blev väl behandlad 'he'
Barnet blev väl behandlat 'the child'