The grammar of the word "which" will not help you answer your question. The definition of the word "respectively" will.
Even the simplest of searches reveals that the two lists can be mapped in a direct one-to-one correspondence, in exactly the orders presented.
In this context, the jobs of task T1 arrive in intervals of 3 milliseconds, the jobs of task T2 arrive in intervals of 7 milliseconds, and the jobs of task T3 arrive in intervals of 20 milliseconds.
The sentence that you've cited says nothing about when each countdown clock starts. I assume that you're looking at GATE2015-1_46 on GATE Overflow. The rest of that question states that all tasks initially arrive at the beginning of the first millisecond. From this, we know that the jobs of T1 arrive at: 1st ms, 4th ms, 7th ms, 10th ms and so on. Jobs of T2 arrive at: 1st ms, 8th ms, 15th ms, 22nd ms and so on. Jobs of T3 arrive at 1st ms, 21st ms, 41st ms, 61st ms and so on.
I repeat, the clocks start at the first millisecond, not the "zeroth". If you don't keep your terms consistent, you can easily run into what programmers call "a fencepost error".
There is one more instance of the word "respectively" in this question. Unless you understand the word "respectively", you will not know how long each task takes to complete an instanced job.
The grammatical ambiguity that I cannot easily resolve is related to the priority of the tasks. Priority and frequency are inversely proportional given the text of the question. But, does a high frequency universally imply a low period, or does the author of the question mistake period and frequency as synonyms? In the business world, I'd ask for clarification. In academia, I'd simply assume that frequency and period are inverse. The inverse of an inverse brings you back to where you started. So, period 20 is the highest priority (lowest frequency), period 7 is a middling priority, and period 3 is the low priority (and the highest frequency).
Any pending job for task T3 is done before any pending job for task T2. Any pending job for task T2 is done before any pending job for task T1. If you don't understand why, I should (at least in theory) suggest you also research the term preemptive. If your search engine is as bad as mine, you might miss the intended implication: that the preemptive thing happens before another, without the other's consent. The highest-priority item, regardless of the order in which it is received, is handled first.
That's as complete a description of the English as you could wish for. Anything beyond this is clearly in non-ELL realms, such as mathematics and programming.