It is very doable to be raised trilingual in Taiwan in Mandarin Chinese (community language), English (the lingua franca of the world), and another mother tongue of one of the parent, if one parent is a fluent Chinese/English speaker and another parent is fluent English/the other mother tongue speaker, and the parents communicate in English.  These kids are likely biracial.  My brother’s family is like that and may even know this young lady.  So, my nephews are trilingual in Mandarin Chinese (and reads 金庸 novels too), English, and Danish.  They homeschooled a couple of years also.  They likely know a number of families whose kids do something similar, including another Taiwanese-Polish family.

I suspect many of these kids have some international experiences, like growing up abroad till around 6 and/or spend summer breaks abroad in the country of the third language (other than English and Chinese).  It helps when there are many international expats living in Taiwan whose children are native English speakers.  It probably helps when the third mother tongue is held in high esteem by the community.  My suspicion is many of these kids enjoyed some type of alternative schooling at some point, rather than attend 12 yeas of regular compulsory education through the Ministry of Education.  There are established homeschool groups that kids can join.

Learning multiple languages while growing up is not difficult, as long as there is a ready community/resources that supports such development and measure proficiency according to multilingual standards.  Many populace around the world do this all the time.  What is difficult in the US for kids to learn English and a category V language is the lack of such ready community/resources, with English as still the lingua franca of the world and the amount of time needed to be develop higher level competency in the category V language early on.  The difficult task for US families is therefore to “artificially” create such community/resources, or the CLE (Chinese language ecosystem) that I talk about.  Obviously, if parents enroll the children in traditional schooling and want the children to “excel” in the various academic and nonacademic activities that other mostly monolingual kids participate in, that put further time constraint in the picture.