They are Agni, Surya and Vayu
Here is the Hindi translation to the mentioned verse by Gayatri Pariwar:
English translation of it would be as follows:
Sadhakas refine their Atman which is Venerable like fire, water and sun, by knowing the supreme light in their antahkarana (inner cause). They excel their Atman with the help of these three forms of Agni. And then after they look earth from everywhere.
This translation matches with Sanskrit commentary by Sayanacharya with a minor change (click on image to magnify):
Here Agni (fire), Vayu (air) and Surya (sun) are considered as three refining.
The interpretation mentioned above can be considered as Yājñikā interpretations. There exists other interpretations also which can be considered Adhyatmika interpretations. For example,
The interpretation by Swami Dayananda Saraswati is as follows:
PURPOT: A person who is pure in body, speech and mind, and takes pure food, he acquires and learns about real heaven after death.
This interpretation matches with that of Damodar Saatvalekar who wrote Subodha Bhashya on Rigveda.
Another interpretation by scholar Tulsi Ram is :
With threefold cleansing of body, mind and soul,
through the purity of thought, word and deed, by
inculcation of right knowledge (jnana), right conduct
(karma) and right prayer and meditation (upasana), I
purify the holy light of the self, thereby lighting the
flame of the spirit in the intelligence with honesty and
sincerity of the heart, and with these foods and
fortifications of the spirit, I redeem the diamond purity
and generosity of the soul, and then watch the immensity
of heaven and earth participating in the Infinity of the
Vedic Sukta Sanmgraha, book from Gitapress translated this verse as follows:
Here three measures are Yajna (sacrifice), Dana (donation) and Tapa (perseverance) or Shravana, Manana and Nididhyasana.
Hence, considering reliable interpretation by Sayanacharya and Dayananad Saraswati whose commentary are most followed and widely accepted, the answer to the question is:
- Agni (fire), Vayu (air) and Surya (sun) according to Yajnika interpretation.
- Mind/thought, speech/word and Karma (deeds) according to Adhyatmika interpretation.