Some examples of 3X ETFs are TQQQ (triples the daily performance of the tech-stock laden Nasdaq 100 index), SOXS (triples the inverse of the daily return of the PHLX Semiconductor index).
It should be noted the SEC rule does not apply for ETNs (for example, GDXU - MicroSectors Gold Miners 3x Leveraged), but this gap is expected to be closed soon.