Hi there,
ICS are a good distance learning course provider. They offer all the GCSE's you require. However, you would be looking at about £250 per GCSE unless it is possible to get a discount for 3 GCSE's. Alternatively can I suggest the City and Guilds Adult Literacy and Numeracy Level 2.
May I ask, is your intention to go on to higher education, because I hold the above qualifications and am currently studying for the Access to Higher Education and the Universities I am considering all accept Literacy and Numeracy Level 2 as an equivalent to GCSE Maths and English A-C. It is just a thought.