Depending what embassy you are applying from, it may be okay to get letters signed by clients explaining the work you do for them and for how long you've been doing it, with their contact information. Also, bank statements dating back six months or so with regular payments from said clients to back it up. I did that in Canada and it worked fine as part of my evidence. I did also have evidence of my company name on my tax forms, and a bit more as well.
They go by the expiration date on the visa. You can enter any time before then. The three months was more of a guideline so you only apply when you need it and they can have appropriate data.
When arriving in Thailand, the officer won't look to deny entry if you don't have an entry stamp within a three month period of the issue date. They will see your DTV and stamp 6 months from arrival date.
There are mixed responses for this depending on which embassy. Although the requirement is supposed to be just the present balance, there are cases where people are asked to provide further documents including 3 months of savings or checking account statements.
Anonymous participant 385 I understand that concern, but as long as you enter the country each time by plane, the Suvarnabhumi immigtation just sees your visa and stamps. I can't imagine a host of questions and surely not asking for proof of your business or courses when you arrive and then denying the 6 months if you didn't bring papers with you.