How to get funding as a third year PhD student for conferences and thesis write-up?
Everyone wants funding. This is not something you can just decide you want and achieve it within a few weeks. It is extremely competitive. 

Generally, at least in the US and portions of Europe, professors obtain the funding and they pay the student. If your advisor or department doesn't want to fund you, you should strongly consider finding another one.

I have never heard of a student obtaining industry funding without substantial evidence of their success (and usually near the end of their PhD) and even then, it is in the form of a fellowship (e.g., IBM Fellowship). You could certainly apply to these but from what I have seen, you usually need to have done internships there and have a strong internal recommendation.

On the other hand, I have known a few students that drop to part-time near the end of their PhD and they start a full-time job somewhere. Then their company reimburses them for their tuition until they finish.

If you look at it from the perspective of a company, why would they want to fund a random student full-time? They are usually looking for late-stage students who are already successful and that they can trust, or they are looking for collaborations with faculty members.

Note: I was funded by a large tech company after a successful internship, however, the money went to my advisor rather than me directly because they said it was "too risky" to interact with a student directly, and preferred to work with a professor (and thus all the benefits and accountability of a universities's research support and legal team).