If you want to check caspase-3 enzyme activity then you can do an assay and take readings in multimode reader. For this protocol you can check this publication.Article The Unintended Mitochondrial Uncoupling Effects of the FDA A...
To detect in gel use 15% gel. Simple western protocol is fine, try using 1x laemmli buffer for cell lysis. Use the antibody which can detect both caspase-3 and cleaved casp-3.
Its not clear whether you want to analyse caspcase-3 activity or the level of procaspase-3 and caspase-3 (active form or cleaved caspase-3).
If you are interested to analyze the caspase-3 activity, then I would suggest to go for ELISA. ELISA for capse-3 activity assay kits are available, follow the company manual protocol that can give you a quick and reliable data.
If you are interested to opt for Western Blot analysis procaspase-3 and cleaved capspase-3 in your sample, then you can go through the detailed protocol published in Methods Mol Biol. 2014; 1133: 141–154.
Caspase Protocols in Mice. doi: 10.1007/978-1-4939-0357-3_9