Can you find the missing number in these patterns?

79,655,2678 billion

1, 2, 3, 6, 11, 20, 37, 69, ___

(1+2+3) 6(2+3+6) 11(3+6+11) 20 (6+11+20) 37 (11+20+37)68 (20+37+68)125

5, 6, 5, 9, 6, 15, 10, 26, 19, 44, 35, 71, ___

1, A, 1, b, 2, C, 3, d, 5, E, 8, f, 13, G, _21__

(1+2) 3 (2+3) 5 (3+5) 8 (5+8) 13 (8+13) 21

1, 3, 6, 9, 18, 23, 46, 53, 106, 117, 234, 247, ___

1, 3×2= 6, 9×2= 18 23×2= 46, 53×2= 106, 117×2= 234, 247×2= _494__

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the first one, each number is determined by the sum of the three previous numbers, (except the first 3).

20 + 37 + 69 = 126

the second one. still working on it.

the third one. fibonacci sequence. the next one is 21. and the letters are just going in alphabetical order but capatilization is alternating.

last one. prime numbers are being added to each number in order, and then doubled. the next one will be double of 247 = 494, and the next number will have the next prime added to it. 13 was the last one. the next one will be... 17. so after 494 is 511