The phrase "a longitudinal study of three" is correct and usable in written English.
It can be used when referring to a research study that observes three subjects or groups over a period of time to gather data on changes or developments.
Example: "The researchers conducted a longitudinal study of three different communities to assess the impact of the new policy over several years."
Alternatives: "a longitudinal analysis of three" or "a longitudinal research involving three".
In 1995, with a grant from the N.I.M.H., Geller began a longitudinal study of three groups of children: those she had diagnosed as having bipolar disorder, using more precise categorical criteria than those specified in the D.S.M.; those with attention-deficit hyperactivity disorder; and a control group of children who had no known behavioral disorders.
A longitudinal study of three Norwegian farms engaged in on-farm diversification was undertaken through farm visits and in-depth interviews.
The transmission parameters were estimated using data from a longitudinal study of three Danish farrow-to-finish pig herds known to be infected.
More recently, Martínez et al. defined a core set of 24 bacterial genera (Martínez et al., 2013) in a longitudinal study of three healthy human individuals during one year, which would constitute a 'healthy' core microbiota set.
By taking a wider measure of wellbeing, companies are improving profits in areas such as increasing employee wellbeing at work as noted in the second annual World Happiness Report: "Harter et al. (2010) found in a longitudinal study of ten large organizations that worker engagement makes a difference to productivity.
Schneider-Kolsky et al [32] performed a longitudinal study of nine normal pregnant women undergoing induction of labour and noticed no difference in Activin A levels in labour in a normal pregnancy compared to pre-delivery values.
The first is a longitudinal study of six Indian nurses, who were interviewed three times over eight months from the date of arrival in the United Kingdom in 2005, and 10 of their managers and mentors.
The LSAC aims to identify the social, familial and individual factors influencing children in their early years of development, using a longitudinal study of two child age cohorts [ 24- 26].
This paper reports on a longitudinal study of how two technological fields that received significant public funding evolved biochemical conversion of biomass and thermal conversion of black liquor.
The rates for scenario ES1 were based on a longitudinal study of 585 BTV-8 confirmed-infected cattle farms [4], whereas the rates in scenario ES2 are based on a longitudinal study of 72 dairy farms [12], [13].
The data were from the Regional sample of the West of Scotland Twenty-07 study, a longitudinal study of health amongst three cohorts, aged 15, 35 and 55 when first studied in 1987/88 (see elsewhere for full details) [ 14- 16].
