GOD is the one who is the Karta (Subject), Karma (Object), and Kriya (Task). He/She is the one who can do all the following three tasks alone viz. Srushti, Sthiti, and Laya. The same statement when i have to put in simple English, i would say the one who does the following things alone is a GOD!!
Believe me, the aforementioned 3 tasks can't be done in an 'Out of Sync' way! For that, a proper synchronization needs to be maintained.
Technically speaking, if you plot a Design Structure Matrix (DSM), and analyse the dependencies, the output would be a Deadlock! As per DSM rules, to resolve a deadlock area, only a single resource should work on those tasks otherwise it can't be resolved. Well, if you don't know what DSM is, it's a methodology (and tool) designed by 'MIT' which takes your work items as input and suggests the best possible execution flow. For more details on DSM visit the website. Let me come back to spiritual explanation rather than being algorithmic here.
The 3 tasks Srushti, Sthiti, Laya are mutually dependent on each other and hence there always exists a deadlock among them. So, as the common belief goes among the Hindu people that there are 3 deities viz. Brahma, Vishnu, and Shiva to take care of these tasks is a Misconception!!
Hinduism mentions about 3 primary deities viz. Brahma, Vishnu and Shiva but essentially they are one and the same!! These 3 names are the 3 attributes of the GOD!
If a river splits into 3 channels and flows in 3 different regions and is called by 3 different names, what difference does it make to us? Essentially all 3 channels are the same 'Water' and originated at the same source! Same theory applies to the Hindu trinity as well. In order to execute the tasks of the universe the same Supreme being attributes himself 3 different names but is non-dual from each other. That is what Advaita also tells about the God.
Brahma primarily creates, but he also sustains by giving boons to the deserving and maintains harmony. So isn't he the sustainer? isn't he doing similar job of Vishnu? Now when he gives unusual boons to Demons, he also creates a trap door in some way or the other to put an end to the demon. Also, when demi-gods seeks his guidance for killing the demon, he reveals them the secret and indirectly helps the destruction of the demon. So is he not doing destruction indirectly? is he not doing the job of Shiva?
Vishnu's prime task is protection, sustenance. But had he not created Madhu and Kaitaba and has he not been protecting the world by killing the sinners/demons? So, is he also not doing the 3 tasks alone?
Shiva's so called primary task is destruction. But it would be an off topic here if i explain the difference between Rudra and Shiva. It's Rudra who destroys, Shiva does everything. So let me say Rudra in this context. So, Rudra's task is to dissolve. As Shiva had he not created Bhairava, Ganesha, Skanda kind of great gods? Has Shiva not been sustaining the universe by taking care of the devotees and protecting them? So is he not doing the 3 tasks alone?
So, now tell me what's the difference between the three deities? They are essentially one and the same as like as 3 different streams of water still remains water only!
So, let me come to a conclusion here that it's only one God who is called by the 3 different names!!
P.S:- People who follow Dwaita perspective (Dualism) they may have a different opinion, and may consider one form as superior to another. Even in that case, the 3 gods converge at a common God, for Vaishnavites he is Vishnu, for Shaivites he is Shiva, for Shaktas, it is mother goddess Shakti (Energy). Also there exists theories given in KMG Mahabharata kind of authentic texts supporting Shiva which tells that it's one god only (Shiva) who exists as/is called Vishnu in Vaikuntha, and Brahma in Satyaloka. A detailed topic is presented on another blog of mine here. In any case, it's only one God with different names/forms..!!!