Hello,
In NEET 2019 the cutoff for DNB radiology was:
Round 1 - Rank 4439 (113 allotted)

Round 2 - Rank 6443 (37 allotted - including vacancies of Round 1)

Extended Round 2 - Rank 7891 (15 allotted - including vacancies of Round 2 )

Round 3 - Rank 10124 (5 allotted - including vacancies of Ext. Round 2)

Note that

These are for UR category (General).

Also, these are NEET PG ranks. DNB releases a DNB specific Merit Position after excluding those who do not apply for DNB. This will be 5% lesser than above rank.

Most importantly the seats in extended round 2 and round 3 are minimal purely based on vacancies and can never be relied upon (should be 5 to 10 seats alloted).
According to previous year data you may get a seat in round 3 but this depends on the number of students taking dnb radiology this year and the number of vacant seats.
Hope this helps.