How easy is it to get a trip to golden triangle from Chiang Rai?

We would land in Chiang Rai around 9am, could we just go to the center and rent a trip to golden triangle or would it be too late? Also, where would be the place to go in order to get one in-situ instead of booking with an agency tour?

And one last question, in the golden triangle are you allowed to cross the borders and be in three countries in one day?

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