The landlord doesn't want to refund my deposit

Hi i have such a problem i moved in and paid deposit to landlord /agent on 15.06.17 for a double room for me and my wife .after a week my fother decided to come to london as well and we already needed a place to live for him in the same house with us, as he doesn't speak english at all. i called the agent to speak about this. she said she have to check if anything available and she will let me know. it passed a couple days -no answer. after that i tried to call the main agent/landlord...no answer...than i texted to him about my question.....no answer again( he never picks up the phone, just one time when i found accomodation). On 25.06.17 we found another accomodation and in the same day i texted to them to let them know about the date we are moving (01.07.17). Next day they called me and said they are not going to refund my deposit as we lived there less than 3 month(thrre is writen in the recep the minimum stay 3 months). So could anybody help me please.It is not serious...they are not serious agency...as they are not picking up the phones and answering to the messages. They are not adisting and helping their customers... Thank you in advance!