- Event X is here. Three years in the making, now it’s your chance to shine at our epic event. (I thought of changing it to: "Three years in the making, this epic event is your chance to shine," but I don't think that quite works.)
First of all there is no participle in the phrase "in the making", and the use of an article before "making" makes that quite explicit. In this construction "making" is a noun.
(Collins) making
(meɪkɪŋ IPA Pronunciation Guide )
plural makings
- uncountable noun
The making of something is the act or process of producing or creating it.
• ...Salamon's book about the making of this movie. [+ of]
• Ducks' eggs are particularly prized for cake making.
Synonyms: creation, production, manufacture, construction
"In the making" in this phrase should not be confused with "in the making" meaning "becoming".
(Collins) in the making phrase
If you describe a person or thing as something in the making, you mean that they are going to become known or recognized as that thing.
• Her drama teacher is confident Julie is a star in the making.
• I think it's a disaster in the making.
Synonyms: budding, potential, up and coming, emergent
So, if something is dangling, it can't be but the modifier "three years in the making". Here is a characterization of dangling modifiers.
(Yourdictionary)
A modifier changes, alters, limits, or adds more information about something else in the sentence. Ideally, a modifier comes right before or right after the word it modifies. When that other word is missing, the modifier becomes a dangling modifier, making the sentence unclear.
(Scribbr) A modifier describes or qualifies another part of a sentence. A dangling modifier occurs when the intended subject of the modifier is missing from the sentence, and instead another subject appears in its place.
Dangling modifiers often take the form of an introductory phrase that is connected to the wrong thing.
(CoGEL Unattached nonfinite and verbless clauses) § 15.59 It is considered to be an error when the understood subject of the clause is not identifiable with the subject of the matrix clause, and perhaps does not appear in the sentence at all.
• *Reading the evening paper, a dog started barking.
• Reading the evening paper, he heard a dog that had started barking.
(grammaring),
As what is being modified is "this epic", and this noun phrase is not the subject of the matrix clause, then, according to the definition, the phrase "three years in the making" is a dangling modifier, but the modification envisaged by the OP does remove this problem and would seem to make the sentence a fully correct one.
This is so because this modifier can be considered as a participial clause from which the participle (past perfect participle "having been") is ellipted, "epic" being its implied subject.
Having been (for) three years in the making, this epic …
Three years in the making, this epic event is your chance to shine.
Sentences with the same construction can be found in the literature. Some instances are found below.
(ref. 1, 2016) Three years in the making, and already covered elsewhere in these pages, The Song Remains The Same finally made it to movie theatres in both the US and UK.
(ref. 2, 1997) Three years in the making, the reassessment evaluated possible health effects from exposure to dioxin, including cancer, reproductive, developmental, immune system, enzyme induction, hormone, and endocrine effects.
(ref. 3, 2016) Three years in the making, Peachtree Golf Club in Atlanta opened nine holes in October 1947 and the additional nine in July 1948.
(ref. 4, 1937) Two years in the making, it includes every penny in the Lincoln series, and every one shines.
(ref. 5, 1994) Two years in the making, Enterprise Objects Framework essentially puts a buffer between NextStep object-oriented developers and relational databases like those from Oracle Corp. and Sybase, Inc., which generate the necessary SQL ...
(ref. 6, 2010) Four years in the making, it is a vindication of the team's hard fought and close second place finish at last year's State Championship and a testament to the spirit of the team.
(ref. 7, 2002) Four years in the making, this album does not disappoint.
(ref. 8, 1997) Five years in the making, Forever finds Brown at his creative peak.
(ref. 9, 2013) Almost five years in the making, the Special Public Hygiene Regulation (1873)
Sex, War, and Disease in the Tropics [55]
was a reflection of the state of both international and domestic debates on the issue of prostitution, public health, and social order in the mid-nineteeth century.