What is the aim? To stablish a degree of the purity of these compounds?

Provided you need to carry out the control of the presence the impurities, please, receive the attached files from European Pharmacopoeia 7th ed. There is a description of GC method to establish alcohol % aldehyde simultaneously. As for the acid, perhaps, the conditions should be the same or close to those for the alcohol

Dear M. G. Meera,

you might analyse your sample using SE 30 stacionary phase in a shorter glass capillary tube /15m long, inner diameter 0.5 mm/ though I would prefer a short 120 cm long glass column of 3 mm in diameter paked with Chromaton N Super or similar filling coated with 3 to 5 % of SE 30. Carrier gas N2 or better He, flow rate of 30 ml/min. FID detector.

Instead of isothermal separation I would recomend you to use programmed increase in the thermostat temperature from some 150  to  260 oC. The temperature gradient could be 5oC/min. 

The question is thermal stability of SE 30. If the temperature limit of this stacionary phase excceds 260 oC your can analyse your samples.   

For information : b.p of benzoic acid is approx. 250 oC, b.p. of benzaldehyde is 179 oC, and of benzylalcohol it is 205 oC.

In the case of lower thermostability of SE 30 you can separate benzoic acid from other analytes with strong anex resin, and  determine benzyaldehyde and benzylalcohol using thermal gradient chromatography as described above. However, with the final analysis temperature should be some 210 oC.

Then wash the anex with small volume /3 to 5 ml/ of 5 % acetic acid and rotavap this solution to dry state at 80 oC /H2O and an excess of CH3COOH will evaporate/. A tiny amount of dry residue would represent benzoic acid. Dissolve  dry residue in diethylether and prepare benzoic acid methylesther via use of diazomethane. Be careful as CH2N2 is a toxic and explosive moiety sensitive to heat, friction, UV light and  shock. Diazomethane is developed in a special glass apparatus and in a glass shilded digestorium. You may use also another alkylation methods published in various books concerning derivatization methods in chromatography.

In my opinion, the best solution for your analyses is use of HPLC with UV or DAD detector.

I use a polar column such as HP-20 (100% PEG) for the analysis, with inlet temperature of 250C and the oven program starting in 150C up to 220C at 10 C/min for 1 minute as final time. If you are using a FID detector, its temperature could be 220C with 40 mL/min of H2 and 400 mL/min of air. depending on the column dimensions (530 µm or 250 µm i.d.) and column length (30 mm for instance), those compounds will be at around 2 min for the benzaldehyde and 4.5 min for benzyl alcohol.