Senjumaru’s feat is approximately 720 megatons.
Earthquake magnitude is a logarithmic scale, so a magnitude 3 earthquake releases about 32 times less energy than a magnitude 4 and about 1,000 times less than a magnitude 5.
Earthquakes naturally dissipate energy as seismic waves travel through the earth, so an earthquake large enough to be felt worldwide would need to start with immense energy at its epicenter.
The shaking felt at any location depends on its distance from the epicenter, the local geology, and soil conditions. A magnitude 3 shake would be light/weak.
Modeling energy transmission through the entire earth is complex. But as a very rough illustration, a 5 megaton TNT detonation (large nuclear bomb) releases about the same energy as a magnitude 6 earthquake. So a worldwide light shake might require significantly more energy, though the TNT equivalent would be highly speculative.
In reality, no artificial explosion on or below earth's surface could actually generate a smooth worldwide earthquake. The released energy would follow existing fault lines and dissipate irregularly.
It's very difficult to provide an accurate estimate of the energy in joules required for a hypothetical worldwide light earthquake shake, as there are many complex factors involved. However, we can make some rough assumptions to get in the ballpark:
Let's assume a magnitude 3 earthquake, which has around 1% of the energy of a magnitude 5 quake.
A magnitude 5 earthquake releases around 1.8 x 10^15 joules of energy (equivalent to about 4 megatons of TNT).
If we assume the energy dissipates uniformly through the earth's crust and upper mantle, the surface area of the earth is around 5.1 x 10^14 m2.
So very roughly, to generate a light shake across that entire area, the energy required would be on the order of 1.8 x 10^15 joules x 100 (to scale up to a mag 3) / 5.1 x 10^14 m2 = around 35,000 joules per m2 across the earth's surface.
Multiplied out over the earth's surface area, that equates to very roughly around 2 x 10^20 joules needed in total energy release to generate a light worldwide shake.
This is a very rough "back of the envelope" estimate that grossly simplifies real seismic wave propagation. But it gives us an extremely high baseline number in the 10^20 joule range for the minimum energy required. The actual number could be orders of magnitude higher when accounting for dissipation through the earth. But this at least gives us an idea of the immense energy involved for a worldwide effect.
1) Earthquake felt in both the skies and earth:
It's not physically possible for seismic waves from an earthquake to propagate through the atmosphere. The shaking feeling in the "skies" would have to be explained by some other phenomenon.
If we speculate some massive atmospheric disturbance in addition to the seismic waves through the earth, it would require significantly more energy. As a very rough estimate, likely on the order of 10x higher, so around 2 x 10^21 joules.
2) Earthquake felt across 3 planet Earths:
The surface area of 3 planet Earths would be about 3 times larger, at around 1.5 x 10^15 m2.
Using the same rough assumptions as before, the total energy required for a light shake would scale proportionally.
So for 3 Earths, very approximately 10^20 joules x 3 = 3 x 10^20 joules.
These are extremely rough estimates meant to illustrate general scales and should not be taken as accurate calculations. The actual energies could be orders of magnitude different in reality.
Oops, made a mistake, my previous estimate for the 3 Earth scenario was off. Let me re-work that calculation:
For a light worldwide earthquake on 1 Earth, we estimated very roughly 10^20 joules
10^20 joules is approximately 240 megatons of TNT energy
For 3 Earths, the total surface area is about 3 times greater
So as you pointed out, the total energy would be roughly:
10^20 joules x 3 = 3 x 10^20 joules
Converting this to megatons of TNT:
3 x 10^20 joules / (4.184 x 10^15 joules per megaton) = 720 megatons of TNT
So the very rough estimate for a light worldwide earthquake felt across the surface of 3 planet Earths would be around 720 megatons of TNT energy.