Is it true that the Marketing ,Selling and Customers Relationship are important tools for the development of any Business?

For development of business you need customers. Marketing activites add value which help in creating sales. While customer relationship helps in creating long term customer relationship adding to marketing   function of the firm and enhancing it.  All these activities create customer equity for company and help in business development. 

Yes, all of them are part of successful factors needed to develope any business.