There are tons of combinations (more than I can list here), so I'll just list some common ones. You can use these to estimate the probability of other combinations, or you can use this calculator: https://deckulator.appspot.com/

Dragons/Thunders/Spellcasters/Waters		Odds of drawing THTD in opening hand (total/Thunder/Thunderless)
6/4/7/14									26.5% / 22.1% / 4.4%
6/5/7/14									30.6% / 26.4% / 4.4%
7/5/6/13									34.3% / 29.5% / 4.8%
9/4/6/12									36.8% /	29.2% / 7.6%
6/7/6/12									37.0% / 33.9% / 3.1%
7/6/6/12									38.2% / 33.9% / 4.3%
8/5/6/12									38.2% / 32.3% / 5.9%
9/4/7/13									38.3% / 29.2% / 9.1%
8/6/5/12									41.9% / 37.1% / 4.8%
9/7/3/10									48.5% / 44.6% / 3.9%
13/4/6/11									49.1% / 35.6% / 13.5%
10/8/0/0									51.7% / 51.7% / 0.0%

Dragons/Thunders/equips						Odds of drawing 3300 THTD in first 9 cards (excluding Thunderless)
6/4/1										10.4%
8/7/1										16.0%
6/4/2										18.9%
6/4/3										25.9%
8/7/2										29.1%
8/7/3										39.6%

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Assuming 30 cards left in the deck, if you have a Dragon and equip in the hand and MUST draw a THTD next turn, it is optimal to toss the Dragon when you have:

12 Dragons and 4 Thunders left
12 Dragons and 5 Thunders left
13 Dragons and 6 Thunders left

(Keep in mind that these situations are rare, and you often have additional turns to draw the THTD. In these cases, fewer Dragons are required.)