Here's one potential way of rewording the sentences to make them slightly more concise:
The flow rates of sweep gas were 0 mL/min (A, D), 3 mL/min (B, E), and 6 mL/min (C, F).
"Amount" is a fine word to use, but here I switched it to "flow rates" to be more specific (similarly, you could use "volumetric flow rates" or other terms). The form that you would choose might depend on what context these captions are in and the convention of the academic field.
And as @Emma notes, here is another possibility/form:
In A, B and C conditions, the amount of sweep gas was 0, 3 and 6 mL/min, respectively, and in D, E and F conditions, said amount [or that amount] was 0, 3 and 6 mL/min.
These two proposed sentences put different emphasis on how the conditions are grouped (and that might influence which form you wish to choose). The first sentence emphasizes how the pairs are similar (A and D, B and E, C and F), while the second sentence emphasizes how the triplets are similar (with A, B, and C being together and D, E, and F being together).