on what the Buddha taught
Sabbe saṅkhārā aniccā
Sabbe saṅkhārā dukkhā
Sabbe dhammā anattā
A common explanation is that since the third line talks about dhammā, instead of saṅkhārā, it includes both saṅkhārā and asaṅkhārā. And Nibbāna, being asaṅkhārā, is thus included. But is this a valid inference?
A stream-enterer has understood anattā, then the last line is clear. Till then anattā is misunderstood (idem the others). To come to understand anattā we need to train. And what is there to train? In this context we can look at SN22.15 where is said that the five masses (khandhā) are impermanent (anicca). And what is impermanent is suffering (dukkha). What is suffering is not-self (anattā) and should be seen with wisdom as it really is: ‘This is not mine, this I am not, this is not my self (attā)’. Which leads to getting fed up and with being fed up comes fading. Through fading, liberation and with liberation there is that knowledge.
That the masses are impermanent is here a given, a statement; it is not explained. And in order to understand we need to learn to see for ourselves how this is so, which requires a thorough grasping of their principle. When we understand the principle of f.i. matter, we can even say something about matter which we have never ever encountered before. To look at this principle we need to look for, investigate, a more general level. What do all manifestations of matter have in common? That what it is, is then its characteristics and all matter behave accordingly. Thus when wood is understood then to that extend all wooden things are understood. Likewise, when saṅkhārā are understood (impermanence) then all dhammā are understood (as not-self). And that is what those three lines say. This way they can be seen as a very compact instruction.
But would that rule out Nibbāna?
By implying Nibbāna as being included, it must also imply impermanence (anattā is seen because impermanence is seen) and with impermanence suffering, which simply can’t be. The line 'sabbe dhammā anattā' does not justify making such (atta or anattā) claims about the asaṅkhāta at all.