My Vivoactive 3 won't charge

Did you try to clean the charging pins and use a paperclip to press them so you feel the spring action ? If it is empty I can see no way to reset it, perhaps pressing the button while applying the charging cable, but without any charge I don't think it works. You can try Garmin support for suggestions perhaps if you open a ticket. 

I did see if little pins and I did see spring action, the triangle appeared, then nothing, won't charge.

So it is receiving power. Try to hard reset it while the triangle is visible by pressing the button until it shuts down. 

i notice also a little loose of the connectors had to force it to stay connected, how can I arranged it

If you need to force it there is something with the pins. You sure they are clean and can be pushed backwards ? Anyway keep it on as long as you can to see if it starts or press the button while forcing the connector..