In other words, It is a power supply with an output capacity of 36W (12V * 3A).
First, Device "A" consumed 12V 1.7A, which is 20.4W.
The power supply still has 15.6W of spare capacity.
Second, Device "B" consumed 5V 2A, which is 10W.
But the with DCDC converter connected.
The converter does not convert voltage with 100% efficiency.
This time about 80% using to calculation.
5V * 2A = 10W
10W / 0.8 = 12.5W
12.5W = 12V * 1.05A
The converter consumes 1.05A from a 12V power supply (12.5W, because 80% efficiency) to output 5V 2A (10W).
Now, The total output of the power is 20.4 + 12.5 = 32.9W.
The current is 2.74A at 12V.
Don't you think this fits in the power capacity you described?