How can an international business access SquareCash, Venmo, or PayPal APIs?

Almost all such services provide access to their APIs via their developer/partner program. I know PayPal does, not sure about SquareCash or Venmo.

However, a word of caution on using these services for your money transfer business. In all three cases, these services prohibit the use of their service is the activity you are engaged in classified as a Money Services Business. Since international money transfer business is classified as an MSB activity, you are therefore prohibited by their Terms of Service (look up under prohibited services) for using their service for international money transfer business.

I did a short video on this, not too long ago:

https://www.youtube.com/watch?v=...