I am a UK citizen. My Thai wife and I have been married for 2 years. She has been to the UK on a Standard Visitor visa (November 2017 to May 2018). Since then, her applications for a visa have been refused three times (two were for visitor visas, one was for a long stay). What can we do now?
As a UK citizen, you can certainly have your wife be with you long term, but attempting to do on a Standard Visitor visa is not a suitable approach. A year ago, in your November 2017 question, you indicated that she had just arrived and would remain a full six months, per the validity period of the visa.
Your question today suggests that, immediately after she left the UK, she applied for the Standard Visitor visa, making three applications, twice asking for 6-months and once for a longer validity period. All three were refused, and three refusals in such a short time can put her in jeopardy of receiving a ban. (And if the 6-month stay was longer then what she asked for when she applied, that may have also been counted against her.)
Stop applying. After three refusals, it is unlikely that she would be granted a visitor visa.
If the plan is for her to join you in the UK, the correct route would be through settlement. Step through these questions to find out if she can apply to settle in the UK; it will indicate what you and she would have to do.
It may help to seek the assistance of an immigration advisor. The UK regulates and registers those legally able to offer such advice and services (advisers, solicitors, barristers and members of approved professional bodies). Here are resources to find guidance: