Here are proven platforms (no particular order) that let startups handle WhatsApp, Facebook Messenger and email from a single, shared/omnichannel inbox — with a short note on strengths and fit for startups.

• Intercom — shared Conversations Inbox with native WhatsApp and Messenger support, plus email & automation (good if you want conversational marketing + support in one product). Requires a Meta Business account for WhatsApp. (intercom.com)

• Front — shared team inbox that brings email, Messenger and WhatsApp into one workspace; strong routing, rules and simple agent workflows (startup-friendly UI; paid plans add WhatsApp). (front.com)

• Zendesk (Support Suite / Messaging) — full-featured omnichannel support: email + Facebook Messenger + WhatsApp (via Messaging/Smooch and integrations); strong ticketing & scalability if you expect rapid support growth. (techforgood.zendesk.com)

• Freshdesk / Freshworks Omnichannel — email + Messenger + WhatsApp support via Freshdesk Omni apps and connectors; simple onboarding and lower-cost tiers for small teams. (freshworks.com)

• HubSpot Conversations (Shared Inbox) — native support for WhatsApp, Facebook Messenger and email in the Conversations inbox; good if you already use HubSpot CRM/marketing. (WhatsApp features may require Marketing/Service Hub Professional or higher and Meta verification.) (knowledge.hubspot.com)

• Twilio (Conversations / Flex) — developer-first omnichannel platform that supports WhatsApp, Messenger (beta/public integrations), email and SMS; highly customizable (better if you have dev resources). (static1.twilio.com)

• MessageBird Inbox — omnichannel inbox built on MessageBird (WhatsApp Business API-based) supporting WhatsApp, Messenger, SMS and email routes; good for global messaging and telephony combos. (messagebird.com)

• respond.io — purpose-built omnichannel messaging inbox for WhatsApp, Messenger, email, webchat, TikTok, Instagram etc.; strong workflow automations and WhatsApp Business Platform support. (respond.io)

• WATI (WhatsApp-first) — WhatsApp-focused platform with team inbox, automations and Zapier integrations so you can connect email/CRM workflows; good for commerce & support teams that prioritize WhatsApp. (wati.io)

• Zoko (WhatsApp-first / commerce) — WhatsApp commerce & inbox platform with integrations (e.g., Zapier/Shopify) so you can funnel messaging and email workflows together; good for Shopify/e‑commerce startups. (zoko.io)

• Missive — collaborative shared inbox that unifies email, WhatsApp, Messenger and other channels with internal threads & assignment features; well-suited to small teams who want email-style workflows plus messaging. (missiveapp.com)

• Help Scout — simple, email-first shared inbox that also supports Messenger (and other channels) via channels integrations — friendly for small support teams that want minimal setup. (docs.helpscout.com)

Quick setup / selection tips

• WhatsApp specifics: most platforms connect via the WhatsApp Business Platform (API) and require a Meta Business account/verification or a BSP (business solution provider) flow — expect phone-number verification, template-message approval for outbound templates, and possible per-message or BSP fees. (Confirm requirements/pricing with the vendor before buying.) (intercom.com)
• If you want minimal dev work: choose Intercom, Front, HubSpot, or Freshdesk (no-code setup + shared inbox). (intercom.com)
• If you need deep customization / global routing / telephony: Twilio, MessageBird or respond.io are more flexible but require engineering. (static1.twilio.com)
• For WhatsApp-first sales/commerce (Shopify etc.): Zoko or WATI specialize in commerce flows and catalogs. (zoko.io)

If you want, tell me:

• whether you prefer no-code vs developer platforms, and
• any must-have features (SLA/SLA routing, CRM sync, Shopify, analytics, price sensitivity),

and I’ll narrow this to the 2–3 best fits and include likely price tiers and setup steps.