Sample Code:

Lightning Component:

<aura:component implements="lightning:recordHomeTemplate">    
        
    <aura:attribute name = "left" type = "Aura.Component[]" />    
    <aura:attribute name = "middle" type = "Aura.Component[]" />    
    <aura:attribute name = "right" type = "Aura.Component[]" />    
        
    <table>    
        <tr cellpadding = "8" cellspacing = "8" style="vertical-align:top;">    
            <td width="14%">    
                <div class="slds-scrollable" style="height:900px;">{!v.left}</div>    
            </td>    
            <td width="1%"></td>    
            <td width="45%">    
                <div class="slds-scrollable" style="height:900px;">{!v.middle}</div>    
            </td>    
            <td width="1%"></td>    
            <td width="39%">    
                <div class="slds-scrollable" style="height:900px;">{!v.right}</div>    
            </td>    
        </tr>    
    </table>    
        
</aura:component>   

Design:

<design:component>  
      
    <flexipage:template >  
          
        <flexipage:region name = "left" defaultwidth = "Medium"/>          
        <flexipage:region name = "middle" defaultwidth = "Medium"/>          
        <flexipage:region name = "right" defaultwidth = "Medium"/>  
          
    </flexipage:template>  
      
</design:component>  

Output: