Is it correct to say that the tilakkhana (anicca, dukkha, anatta) is ultimate reality?
Tilakkhana is Pali for "three lakṣaṇa". "Lakshana" means indication, symptom, attribute, feature, omen, that by which a thing is recognized.
Anicca, dukkha, anatta are the three aspects of all dharmas or all phenomena. Dharmas have many different attributes or qualities, but these three are the subset of qualities that helps us let go of our simplified assumptions about dharmas and to realize their true mode of existence aka "ultimate reality".
The way dharmas really exist does not come in three pieces. Anicca is not separate from dukkha is not separate from anatta. Instead, the ultimate reality is this fluid stuff. We, being equal parts naive and fearless, try to describe this stuff in terms of distinct dharmas, but every time we create these distinctions they are inaccurate, unreliable, and unstable - and that's what trilakshana refers to.
So it would be more accurate to say that trilakshana is a characterization of our description of reality than something that applies to reality itself.
In other words, trilakshana is Samsara we overlay on top of Nirvana - not Nirvana itself which is free of the three marks.
Nibanna has not the tilakkhaṇna marks. Nibanna has only the not self part and it is as real as whatever has the 3 marks. But nibanna is not a product of mano, ie an idea, not an object of the 5 usual senses, and not a material good, so it is a weird object which is why the buddha uses ''direct knoweldge'', instead of the ''intellectual knowledge'' that philosophers claim exists and actually is knowledge, to talk about the knowledge of nibanna.
AN 3.136 explains the tilakkhana (anicca, dukkha, anatta) are invariable natural laws; even when they are not known or perceived.