Bài 1:
Gọi số tự nhiên cần tìm là ABCDE.
Theo đề bài, ta có phương trình:
10^5 * A + 10^4 * B + 10^3 * C + 10^2 * D + 10 * E = 3 * (10^6 * A + 10^5 * B + 10^4 * C + 10^3 * D + 10^2 * E + 2)
Simplifying the equation, we get:
7 * (10^5 * A + 10^4 * B + 10^3 * C + 10^2 * D + 10 * E) = 6 * (10^6 * A + 10^5 * B + 10^4 * C + 10^3 * D + 10^2 * E) + 6
Simplifying further, we have:
10^5 * A + 10^4 * B + 10^3 * C + 10^2 * D + 10 * E = 6
This equation implies that A = 6, B = 0, C = 0, D = 0, E = 0.
Therefore, the natural number with five digits satisfying the given condition is 60000.

Bài 2:
Gọi số tự nhiên cần tìm là ABC3.
Theo đề bài, ta có phương trình:
10^2 * A + 10 * B + 3 = 10 * A + B - 1992
Simplifying the equation, we get:
99 * A - 9 * B = 1995
Dividing both sides by 9, we have:
11 * A - B = 221
The possible values for (A, B) are (20, -1), (31, 10), (42, 21), (53, 32), (64, 43), (75, 54), (86, 65), (97, 76).
However, since we are looking for a natural number, the only valid solution is (97, 76).
Therefore, the natural number with the given condition is 9776.

Bài 3:
Gọi ba chữ số cần tìm là A, B, C.
Theo đề bài, ta có phương trình:
100 * A + 10 * B + C = 100 * B + 10 * C + A + 198
Simplifying the equation, we get:
99 * A - 90 * B - 9 * C = 198
Dividing both sides by 9, we have:
11 * A - 10 * B - C = 22
The possible values for (A, B, C) are (2, 0, 0), (3, 1, 1), (4, 2, 2), (5, 3, 3), (6, 4, 4), (7, 5, 5), (8, 6, 6), (9, 7, 7).
Therefore, the three-digit numbers satisfying the given condition are 200, 311, 422, 533, 644, 755, 866, 977.