This might sound like an easy question but it is quite difficult in practice. You want to treat compounds that can have widely differing structures and properties as though they are the same. But they are not the same. That means that your answer will be wrong but it might be close enough for your needs. If you really want to quantify, then you need to separate and quantitate each component. But here are a few papers that might help.

Quantification of the Total Phenolic and Flavonoid Contents, and Antibacterial Activity of Dioclea Reflexa (http://www.sphinxsai.com/2014/ph_vol6_no6/2/(1875-1882)%20014.pdf) 

Estimation of total Terpenoids concentration in plant tissues using a monoterpene, Linalool as standard reagent. (http://www.nature.com/protocolexchange/protocols/2529#/procedure)

Qualitative and Quantitative Analysis of Phytochemicals in Five Pteris Species. (http://www.ijppsjournal.com/Vol5Suppl1/6258.pdf) 

Sodium borohydride/chloranil-based assay for quantifying total flavonoids.(http://www.ncbi.nlm.nih.gov/pubmed/18798633)

Determination of total phenolic and flavonoid content, antimicrobial and antioxidant activity of a root extract of Arisaema jacquemontii Blume (http://www.sciencedirect.com/science/article/pii/S1658365514001046)

Estimation of total alkaloid in Chitrakadivati by UV-Spectrophotometer (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3644759/)

A new spectrophotometric method for the estimation of total alkaloids in the stem bark and seed of Holarrhena antidysenterica (Linn.) Wall. and in the Ayurvedic formulation, Kutajarishta 

Studies on phyto chemical constituents, quantification of total phenol, alkaloid content and In-vitro anti-oxidant activity of Coccinia cordifolia (http://www.ijplsjournal.com/issues%20PDF%20files/oct2011/11.pdf)

(Also look up Dragendorf's reagent and Folin–Ciocalteu reagent for more on alkaloids and phenolics, respectively.)

Total flavonoids from a leaf extracts (water or alcoholic extract) can be estimated using one standard method described in Romanian Pharmacopoeias, demonstrated with an accuracy of plus/minus 5%. (http://www.degruyter.com/view/j/chem.2015.13.issue-1/chem-2015; Reference [14] Romanian Pharmacopoeia, 10th edition (Medicala, Bucharest, 1993)

Briefly, three aliquots of 25 – 100 μL vegetal extract are treated with 600 μL of (2.5% w/v) AlCl3 and 1000 μL of 10% (w/v) CH3COONa then accurately finished at 5000 μL with (50%, v/v) ethanol/methanol solvent. A control sample containing 25 – 100 μL vegetal extract and 1600 μL distilled water accurately finished at 5000 μL with (50%, v/v) ethanol/methanol solvent is also made.  Mixtures are incubated at room temperature for 30 minutes and the absorbance of reactions, face to control sample, at maximum wavelength (λ) measured. Total flavonoids content is further estimated using rutin or hyperoside (in the specific case of samples with maximum wavelenght over 420nm) standard calibration curve (using the same procedure) and the results expressed as g total flavones (rutin or hyperoside equivalents) per 100 mL/g vegetal sample.

Semi-quantitative methods for quantifying these phytochemicals are available here:

Alkaloids: http://www.biomedcentral.com/1472-6882/11/3

Flavonoids, phenolics: http://www.sciencedirect.com/science/article/pii/S2212429214000121