And he declared the company has a "sizeable war chest" it is preparing to release for investments in the £2 million to £30m range.
The entrepreneur highlighted the company's intention to ramp up its "deal flow" as the improving economy makes it harder to find investments priced at "low multiples" it was able to pick up during the downturn.
Souter Investments generated a net return of 21% per annum after increasing its investment activity over the three-year period, with the company maintaining its focus on private equity.
The company, which has seen the value of its portfolio increase by 159% since it started six years ago, made eight major private equity investments in the transport sector. It has retained a core holding in Perth-based Stagecoach, which has brought a return of 11% per year over six years, and is an investor in bus manufacturer Alexander Dennis.
Beyond transport, the company made nine investments in sectors such as financial services, telecommunications and recycling.
Souter Investments also notes in its 2013 Investment Review that it has made successful exits from esure, the insurance company which floated in March, luxury yacht maker Sunseeker International, Argent Energy and Wireless Infrastructure Group.
The investment firm is thought to have raised tens of millions of pounds after Sunseeker was sold to Chinese conglomerate Dalian Wanda Group for £320m in June. It had been the largest shareholder in Portofino Yacht Investments, which had a controlling stake in Sunseeker, with Sir Brian having made a personal investment of £9.5m for a significant minority stake in the manufacturer in 2010.
The review highlights that since year-end the company has invested in Irish book publisher CJ Fallon, road traffic accident claims emergency company Winn Solicitors and, subject to regulatory approval, Investment Solutions, a fund administration platform.
Sir Brian said: "We held our nerve throughout the recession, took some brave decisions, invested through the cycle and benefited from having access to capital when many investors were avoiding risk.
"As the financial situation improves it will be more difficult for us to find deals priced at the low multiples we paid during the depths of the recession. To counter this, we are now seeking and finding a substantial increase in deal flow and we have a sizeable war chest to invest in the right opportunities.
"Looking forward we are confident that Souter Investments will continue to invest in successful companies, however replicating the returns achieved in the last three and six years will be a significant challenge."
The Souter Charitable Trust has donated about £45m thanks to the investment company's returns.