Actually, these three versions do the same things what an antivirus should do but only differences are their "target markets" and "few additive features", as per product's version.
Antivirus Plus is for those users who use stand-alone PCs and don't connect their PCs to internet or any kind of network or they connect to internet sometimes but not frequently. This antivirus version offers basic protection against malware infections like viruses and spywares, even it scans and cures internet threats but not like in that specific way as internet security version performs.
Internet Security version is for those users who use internet very frequently as this version is more detailed and diversified one against viruses and all kinds of malware infections. The version offers protection against trozans, worms, backdoors, rootkits, adwares, phishing sites etc. along with basic protection of a PC. One more thing is this version protects a PC from internet threats but not from those infections coming from any local network e.g. LAN, WiFi, Bluetooth etc.
Total Security, as the name describes itself, this version is the most prolific solution. Though, it is the most costly one. The version offers protection and services to a greater extent, if compared with both of two other versions. Besides, this version may offer few more advantageous facilities like PC tuner or data backup module as additional features.
Overall, you can see that all three home editions vary a bit from each other and that difference is either upgradation of security module or implementation of some additional features. Every person chooses the product as per requirement and approach that what suits well.
Well, thank you. Hope the answer could help you.