Having written for all three of the above named websites, I can hopefully give some insight to those who are new to the game of writing website content. Perhaps you have already tried one, found it unsatisfactory and moved on to another in the vain search for cash. I have been writing for Helium since January 2008, Associated Content since May 2008 and Triond for only the past few weeks. Here is my view when looking at all three:
Helium is the place where I have written most of my articles and has given me the greatest financial return. When I first joined Helium, they had just started running a Rewardathon where writers were paid per article based on the number of writing stars they had as a result of the rating system in place. The rating system is by no means perfect but the articles generally find their true place after a while. Combined with the Rewardathon earnings, I was also able to sell a few Marketplace articles and won or placed in the weekly writing contests. Without those means of earning available, I would not have earned much simply relying on page views.
Associated Content provided a little bit more on page view earnings but is not the ideal arrangement if you happen to live outside the USA. As a US resident you can earn up front payments, but if not, you have to rely on page view returns which of course is limited. Having submitted articles for a few months, I have been given some idea of the kind of return that can be expected. I certainly wouldn't give up my day job to pursue this as a means of income.
I am relatively new to Triond having written around 30 articles in the past two weeks. I can see a steady increase as cents are added daily but will obviously need a few months of activity to make a genuine comparison. The difference between Triond and Helium is that Triond will apparently also include a share of ad revenue, although I understand this is only if the click through results in a sale. My general feeling is that it will take a high volume of articles to see a reasonable financial return but time will tell.
Comparing all three from my own perspective, Helium has definitely proved more rewarding financially because of the other opportunities to earn apart from reliance on page views. My guess would be that Helium, Triond and Associated Content will all pay pretty much the same when it come to page views but then again, I'm sure there rae some writers out there who would beg to differ.