Just wanted to know how can I check to see if Tor is connected through bridges I've defined in torrc?
UseBridges 1 option makes sure Tor only connects to bridges. Checking this is difficult. Use a bridge firewall ( corridor ) and/or a network analyizer.
I added 3 bridges in torrc but still the 3rd option in "Whonix connection wizard" (Tor is censored or dangerous in my area) is disabled.
This is expected. That option is not implemented. It only shows a tooltip when you hover over that option by mice. That's all.
I've tried running tor via first option
That is fine after you added bridges to torrc. However, all it does is commenting in
DisableNetwork 0 in torrc.
and checked Arm to see if bridges working but it seems to me that there is a problem as it shows bridge available but get bridge line didn't parse error.
You need to post the full message [with IPs / ports redacted] as it is difficult to make head or tail of this otherwise.
Don't worry too much about arm, it has many usability issues: https://www.whonix.org/wiki/Tor_Controller#Arm_FAQ
How can I be sure that my connection is through one of the pre-defined bridges?
Dunno what you mean by pre-defined bridges.
how can I make the 3rd option in Whonix connection wizard available?
(as explained above why...) You cannot.