As a quick summary to the information provided by @Traggeter & @Maldoror above, it comes out to:
3*: 74.5%
4*: 19.5%
5*: 6%
EDIT: this is for a total of 472 pulls across 2 separate accounts. With approx. 500 pulls, I should expect these totals to be quite accurate. They also match previously reported pulls to a decimal - including those reported as far back as a year ago. 
From august 22th all 3* or 4* duplicates.
Total number and numbber of pulls or percentages would be helpfull.
52 pulls
34/3*
12/4*
6/5*
Kadilen, marjana, magni, khagan, horgall, quintus
3* - 62 (79.48%)
4* - 14 (17.94%)
5* - 2 (2.56%)
78 pulls total to date.
pulled Hawkmoon
I didn’t track how many pulls I did but Im feel kinda bugged 'cause the last about 15 pulls gave me 7 Tiburtus.
Maybe i was too happy with my results from TC20.month ago I got my last 5* hero(Khagan) now,only feeders 3*…really sad. My heart is bleeding 
I just wish to defer from your conclusion, I am a C2P and I can say I have about 19 of the available 22 regular 4* heroes not including Atlantis and dubs, and all the regular 3* heroes again not including Atlantis heroes and dubs
And have just 3 X 5 * heroes.
From the finding in this thread, TC 20 have better chance of giving out 5 * heroes than any regular epic, legendary or event summons, and it’s been proved here with just a few data samples.
TC20 is not for 3 * heroes perspective as you stated, just like Legendary, Epic or event summons are basically not for 3* heroes, but chances are combined to create a Gacha effect.
However, for me I will run TC20 with this 5% chance as long as it takes to get all missing regular 5* heroes. The 5% chance is far higher than 1.3-1.5 % in the regular summons. And by the way the 3* byproducts come as fodders, double advantage, it only cost hams and recruits not gems, and save gems towards event heroes and HotM.
31 heros
25 3* (Ishtak last pull)
3 4* (Boldtusk Scarlett chao )
3 5 * (Elena Sartana Horghall )
Tyrum
Total pulls: 6
5* - 0
4* - 2
3* - 4
I’ll be starting my first ever TC20 training today so will update with who I get.
My money is on Valen…
Any defense debuff hero is golden for titans 
Already got him full level and had about 3 since haha. Plus full level Grimm (which I pulled another of today ina summon!)
Carver
Total pulls: 7
5* - 0
4* - 2
3* - 5
Today I got Domitia and Isarnia both from TC20s. I would say a jackpot but I don`t need purple. I need blue but not Slow, as I have got Vivica and Quintus which means my third slow… But I’m F2P so it’s not bad anyway.
319 traings
60 blue
92 green
52 purple
59 red
56 yellow
241 3* (17 Berden)
60 4* (6 little john)
19 5* (3 justice, 2 azlar, 2 leonidas, 2 horghall, 2 kadilen, elkanen, lianna, quintus, richard, isarnia, magni, thorne & khagan)
Hawkmoon
|Total pulls|8|:pleading_face:|
|—|—|—|—|—|—|—|
|5 ★★★★★|0|
|4 ☆☆☆☆|2|
|3 ⭑ ⭑ ⭑|6|
|5 ★★★★★| Pull Number | Date |:( |
|—|—|—|—|—|—|—|
|-|-|-|-|
Results from 52 tc20 drops
3* 33
4* 13
5* 6: Justice, Quintus(2), Lianna, Leonidas, Magni.
SG has been good to me for these drops. They balance it with horrible chests in the A tier from Titans. I can’t complain abiut B & C tiers since I did not perform.