Is it permissible to combine the intentions for the Sunnah prayer of ablution, the prayer of greeting the mosque, and the Duha prayer into two rak'ahs, and will the worshipper be rewarded for each intention individually or for only one intention?

The Sunnahs of Wudu (ablution) and Tahiyyat al-Masjid (greeting the mosque) are among the Sunnahs that permit sharing an intention with other prayers, such as Salatul Duha (forenoon prayer). So, whoever prays them with a single intention, that suffices him, and he attains the basic reward. This differs from Sunnahs that are ends in themselves, like the regular (ratibah) supererogatory prayers. However, if one prays each of these prayers with an independent intention, it is better and brings greater reward.