apparently you can convert 3.5mm to speaker wire. however i do agree it is not the best option you have.
soundcards are most likely capable of powering small speakers via the 3.5mm L/R channel but anything more than a very small driver is most likely not going to give you any output without an amp. there are small amps like the fiio which could boost the power on a single 3.5mm but this is normally used to drive high end headsets over 3.5mm not speakers.
an amp with a a digital signal from your pc and actual binding posts for the speaker wire is the best solution and is what you should go for if you want a good solution that is known to work without issue.