The figures showed that St James’s Place received 57 complaints and Openwork had 66.
Sesame saw an increased number of complaints from the last six months of 2017, from 88 to 103.
But the number of complaints upheld against the firm fell from 30% to 12% in the first half of this year.
The proportion of upheld complaints regarding St James’s Place fell from 29% to 15%, with those about Openwork falling from 47% to 23%.
The fall came despite a rise in the number overall of complaints about advisers.
FOS received 979 complaints about financial advisers in the first six months of 2018.
The figure was the first time complaints about advisers had risen in two years.
Elsewhere in the data, Aviva was at the top of the list for pension complaints with 291 and Barclays received the most investment complaints (476).