BUENOS AIRES, May 20 (Reuters) - Argentina's stock, currency and bond market trends on Wednesday.

* The MerVal stocks index rose for a third session, climbing 1.09 percent to 1,550.15 points but trimming earlier gains as profit-taking saw Wall Street turn lower.

* The index is up 21.6 percent since May 1 and has risen 7.75 percent this week.

* Strong crude prices helped MerVal heavyweight Tenaris (TENA.BA: Quote, Profile, Research), which makes steel tubes for the global energy industry. It rose 2.2 percent to 55.2 pesos a share.

* On the broad stock market, volume was thin at $15.9 million. Of active issues, 41 advanced, 24 declined and 13 were unchanged.

* Prices for government bonds traded over-the-counter in Buenos Aires rose 2.5 percent on average, accumulating gains of 15 percent since the start of the month.

* The dollar-denominated Discount bond ARDISCD=RASL rose 4.6 percent, according to the ask price.

* In formal exchange between banks, the Argentine peso closed 0.07 percent weaker at 3.73/3.7325 per dollar ARS=RASL due to central bank dollar purchases aimed at gradually weakening the peso, traders said.

* In informal trade between foreign exchange houses, as measured by Reuters, the peso dipped 0.2 percent to end at 3.7925/3.7975 ARSB=. (Reporting by Jorge Otaola; Writing by Helen Popper)