1. You should find id of this category (my example is for category 105)
2. open your customer/main/products.tpl
Code:
{if $config.Appearance.products_per_row && ($featured eq "Y" || $config.Appearance.featured_only_multicolumn eq "N")}
and replace it with
Code:
{if $config.Appearance.products_per_row && ($featured eq "Y" || $config.Appearance.featured_only_multicolumn eq "N") || $current_category.categoryid eq "105"}
3. open your customer/main/products_t.tpl replace
Code:
{list2matrix assign="products_matrix" assign_width="cell_width" list=$products row_length=$config.Appearance.products_per_row}
with
Code:
{if $current_category.categoryid eq "105" && $featured ne "Y"}
{assign var="per_row" value="3"}
{else}
{assign var="per_row" value=$config.Appearance.products_per_row}
{/if}
{list2matrix assign="products_matrix" assign_width="cell_width" list=$products row_length=$per_row}