Feasible usually refers to an activity that can be achieved in a particular situation. Is it feasible for me to learn Thai. Yes, it is given my ability and resources at hand. Practical is used to describe whether or not that activity will be useful in that situation. It may not be that practical to learn Thai if I do not have friends or language partners interested in helping me with Thai. It would be impractical to learn this language if I have limited resources. I have not heard practicable used that much but it refers to whether something is usually done that way or refers to a norm. It would be practicable for a Thai learner to join italki and look for some language partners to help learn this language. Nowadays with skype and youtube, learning Thai only through books would be a relatively impracticable and outdated method of learning.